On my gentoo box, the only thing I needed to upgrade was modutils. Your mileage may vary for other distros, so make sure to read the kernel changelog and update any packages required! I think some distros (RedHat ?) have binary kernel targets for 2.6.0 already
It is possible to install a Red Hat packaged 2.6.0-test kernel (see http://people.redhat.com/arjanv/2.5/), however some things have changed between 2.4 and 2.6 that can't just be worked around by installing new packages. (USB module names, some mount points, that kind of thing.) If you want a clean boot you will have to change some of the init scripts, and this will break booting 2.4. So it's a bit all or nothing at the moment, and I recommend people who aren't convinced it will do everything they need it to do (I couldn't get my network card working under 2.6) stay on 2.4 until it's released proper. See http://www.wlug.org.nz/KernelNotes for some details. The pages with the specific detail on them are http://thomer.com/linux/migrate-to-2.6.html and http://www.fearthecow.net/index.pl?section=guest&page=kernel Craig