There’s a question about the installer. I confess complete ignorance regarding debian-installer, but Anaconda for Debian has been doing LVM for a while now, on the root or just about any other partition. Once you get the system installed (either via native installer support or via some command-line manual setup), Debian’s infrastructure seems to have no difficulty. This is a configuration Progeny tests fairly regularly, so I’d expect everything to be OK.
Of course, one should not put /boot under anything besides a regular partition, which means that /boot should be mounted if the root is in a LVM. I’ve also heard that root LVMs under Linux are a pain to maintain. Nevertheless, there should be nothing Debian-specific about recommendations regarding the use of LVM.
UPDATE: Andrew Pollock reports that debian-installer supports LVM just fine, and complains about being forced to use LILO instead of GRUB as the boot loader. Actually, that’s one of the benefits of /boot being on a regular partition. I think it’s safer to keep all boot information outside systems like LVM as a general rule; getting GRUB back is a nice additional perk.