Dell has started releasing details about their upcoming Linux offerings, on their Direct2Dell.com site. It covers limitations (both hardware and software), as well as what peripherals will be supported/sold.
Dell will be offering Ubuntu 7.04
According to Jeremy’s Blog on LinuxQuestions.org, they will only be offering Linux on the following systems: Dell E520, 1505 and XPS 410. No mention of any Dell ultraportable laptops.