Yeah, I'm a few days late on this. Heck, Dell had launched the systems on Thursday. Actually, this hasn't been too new, as Dell have been selling Red Hat equipped servers for some time (And they even have their own customized Yum repositories for hardware-specific drivers). But this is a very good sign for Linux (and other open source systems) when Dell is willing to sell systems with Ubuntu 7.04 preinstalled!
Even more importantly than the chosen distribution is Dell's efforts to offer hardware that have open-source drivers. With the right hardware, the end-user can choose to install any Linux distribution and be reasonably sure that it will work without issues. This may not be as big a deal with desktops where you can build your own system and be sure that you use compatible components, but users generally don't have that option when it comes to notebooks. Again, people looking for Linux notebooks too have the option of getting one from a company like System76, but they do not have the pull with hardware manufacturers to release open source drivers or at least open specs for their hardware like Dell would. Some people have theorized that this move by Dell is what pushed AMD to promise open-source drivers for their ATI graphics cards.
There's quite a few sites to check out, like Dell's open source page, Dell's Linux page, or even Ubuntu's page on the deal with Dell. They have two desktops (the Dimension E520n and the Dimension XPS410n) and a notebook (the Inspiron E1505n) available with Ubuntu installed. Here's the link to the systems on Dell.
I'm going to look into getting the Inspiron soon; hopefully, my employer can order one for me in the next week. Now it's a tough choice between equipping the notebook with the Intel® Graphics Media Accelerator 950 (which should have open-source drivers available for it) or the 256MB NVIDIA® GeForce® Go 7300 TurboCache™ (with uses proprietary drivers, but has dedicated video RAM and should have better overall performance for 3D)...