CPU: Intel Core Duo T2400 1.83 GHz	Yes.To enable SMP see below
Hard Disk: 80GB (5,400 rpm) SATA Hard Drive	Yes
Graphic Card: Intel Graphics Media Accelerator 950 (Intel GMA 950)	Yes
Screen: 17`` Wide Screen WXGA+(1440x900)	Yes with 915resolution hack
Wireless Card: Intel PRO/Wireless 3945ABG Network Connection (ipw3945)	Yes
Sound Card: Intel HDA. Mixer: SigmaTel STAC9200	Yes
Modem: Conexant HDA D110 MDC V.92	Not tried yet
Dell Wireless 350 Bluetooth 2.0 Module	Yes
Hotkeys	Working with GNOME only

The installation of the new Ubuntu release 6.06 LST on this laptop was very straightforward. I used the live-cd installer and had to enter in "safe graphics mode". After the live cd finished loading I clicked on the "install" icon on the GNOME desktop, and this started the graphical installer. Following the installer's procedure the installation was complete in less than 20 minutes.

Screen Resolution Issue

After rebooting the only problem was getting the right screen resolution of 1440x900. The problem was easily solved installing the 915resolution patch. I downloaded it from the ubuntu packages repository and installed it with
dpkg -i 915resolution_0.5-1ubuntu6_i386.deb
after rebooting I got the right screen resolution of 1440x900 at 60 Hz Vertical Refresh Rate without having to manually edit any configuration files.

SMP

If you want to get both CPU cores working, you need to install the SMP (symmetric multi-processing) kernel, you can easily do this installing the linux-686-smp package after the default installation, (the system works as well with one core only) using apt or synaptic.

Wireless

Wireless networking worked out of the box!

Other resources

http://www.linlap.com/wiki/Dell+Inspiron+9400

Old Installation Report on Ubuntu Breezy

CPU: Intel Core Duo T2400 1.83 GHz	Yes.To enable SMP see below
Hard Disk: 80GB (5,400 rpm) SATA Hard Drive	Yes
Graphic Card: Intel Graphics Media Accelerator 950 (Intel GMA 950)	Only VESA driver at the moment
Screen: 17`` Wide Screen WXGA+(1440x900)	Yes with 915resolution hack
Wireless Card: Intel PRO/Wireless 3945ABG Network Connection (ipw3945)	Yes
Sound Card: Intel HDA. Mixer: SigmaTel STAC9200	Yes, with some remaining problems (see below)
Modem: Conexant HDA D110 MDC V.92	Not tried yet
Dell Wireless 350 Bluetooth 2.0 Module	Yes
Hotkeys	Working with GNOME only

Installing Linux on this laptop was difficult at first, I initially tried Debian Sarge, and Fedora Core (IV and then V), but with both something didn't work "out of the box", and I wasn't able to configure it by myself (with Sarge, the installation of the base system went smoothly, but the computer got stuck trying to install the desktop environment; with Fedora I couldn't get the native screen resolution (1440x900) and the sound didn't work). I then tried Ubuntu Breezy, and though not everything worked "out of the box", with some post-configuration I've been able to set up a system I'm very satisfied with. Most of the issues I've had won't probably show up at all with the next release of Ubuntu (Dapper), which is scheduled in June.

Disk Partitioning

I decided to go dual-boot, so I resized the Windows partition with QtParted , using the SystemRescueCd . The Windows partition came as NTFS, but QtParted had no problem resizing it, when I rebooted into Windows, it complained for something and did a disk check, but that was it. Note that resizing with QtParted may be lengthy process (it took more than half an hour I think), and at some point it looks like it's frozen (can't click on anything on the GUI), but it is actually working, so wait...!

Installation

I used the Ubuntu live DVD to install, although the Ubuntu installer doesn't have a GUI it's pretty easy to use, and everything went smoothly here.

Post-Install Configuration

Video Card and Screen Resolution Issue

After rebooting I couldn't get in X. To fix the problem I run:
dpkg reconfigure xserver-xorg
choosing VESA as the video card driver, and was able to login graphically but only with a screen resolution of 1024x768. To get the native screen resolution of 1440x900 I downloaded the 915resolution hack directly from its developer page as a Debian package (the Ubuntu package for Breezy was outdated and didn't support the latest Intel chipset. I then installed it with:
dpkg -i 915resolution_0.5-2_i386.deb
and put this line in /etc/init.d/bootmisc.sh
/usr/sbin/915resolution 3c 1440 900
in this way the 915resolution hack is called every time the system boots up, and I'm able to get the native resolution.

SMP

If you want to get both the CPU cores working, you need to install the SMP (symmetric multi-processing) kernel. You can just do this after the default installation, (the system works as well with one core only) with apt or synaptic. Note however that you first need to disable the PCMCIA modules otherwise the system will hang. You can easily disable the PCMCIA modules using BUM (Boot Up Manager).

Wireless

The official Linux driver for the Intel PRO/Wireless 3945ABG has been released, you can get it either from the Intel page , or from the development website. For quick installation instructions see here . There's a script made by wheelspin to install the driver in Ubuntu, you can fetch it on this page . The installation is very easy with this, the only problem I had was that I had to manually copy the firmware in /lib/hotplug, because the directory in which the script put it wasn't OK.

Sound

The sound worked after the installation, but was distorted, I found a solution for the problem on this forum . It says to add this line:
options snd-hda-intel position_fix=1
to /etc/modprobe.d/options, it worked for me, the sound is no longer distorted.

I have one remaining problem with sound, when I plug in the headphones, the internal speakers are not muted, I haven't been able to fix this yet. I think I'll wait for Ubuntu Dapper for it!!