This turns out to be a very complicated issue.  The background is that when nouveau KMS support was added recently, this came by means of having xserver-xorg-video-nouveau depend on some backported kernel modules.  Here's an IRC discussion from today:

16:51 <cjwatson> bryceh,pitti: problem: ubuntu-desktop now transitively depends on grub-pc - ubuntu-desktop -> xserver-xorg-video-all -> xserver-xorg-video-nouveau -> linux-backports-modules-nouveau-lucid-generic -> [...] -> linux-image-* -> grub-pc (recommends)
16:52 <bryceh> cjwatson, ew
16:52 <cjwatson> bryceh,pitti: this is bad because it subverts the installer's logic for boot loader installation and causes bug 526422.  we've always tried hard to keep linux out of the desktop task for this reason.  what can we do about this?
16:52 <ubottu> Launchpad bug 526422 in grub-installer "Grub installer asks too many questions to the user" [Undecided,Confirmed] https://launchpad.net/bugs/526422
16:53 <pitti> cjwatson: (lagging, desktop meeeting going on)
16:53 <bryceh> cjwatson > #ubuntu-x 2>&1
16:53 <pitti> cjwatson, bryceh: would it be prudent to drop the video-nouveau -> lbm dependency?
16:53 <pitti> the other video drivers don't depend on the kernel either
16:53 <pitti> of course that'd require them to cleanly fail to load if the module isn't present
16:53 <bryceh> pitti, but then would l-b-m-nouveau not be included in the cd?
16:54 <pitti> bryceh: we'd need to seed it explicitly somewhere, I guess
16:54 <cjwatson> but then that would have the same problem
16:54 <pitti> or make it a recommends of linux-image in linux-meta
16:54 <bryceh> as long as that's done it'd be fine.  afaik that's the only reason we put the dep on it in there
16:54 <cjwatson> I don't see a clean way to do this, short of making the kernel image package *depend* on lbm-nouveau
16:55 <cjwatson> a recommendation would be ineffective in this case
16:55 <cjwatson> the installer ignores recommends when installing the kernel package, and has to do so because otherwise it would pick up boot loaders when doing so
16:56 <pitti> cjwatson: so the actual linux-image-* -> grub-pc (recommends) should stay and is intended?
16:56 <cjwatson> but I would expect that having the kernel image package depend on lbm-nouveau would cause a chicken-and-egg problem on ABI transitions
16:56 <cjwatson> pitti: yes
16:56 <cjwatson> it's recommends: grub-pc | <other boot loaders>
16:56 <cjwatson> and it got lifted to that due to another bug, iirc
16:57 <cjwatson> apw: ^- is there any chance that we could just have lbm-nouveau integrated into the main kernel package?
16:57 <cjwatson> I wonder if there's any seed trickery I could do
16:57 <pitti> short of that, the only way I see is to make it a dpeendency of -generic, but not of -server
16:57 <pitti> which should amount to the same thing than integrating nouveau into -generic itself?
16:57 <cjwatson> but lbm-nouveau can't be built until after the main package is built
16:58 <pitti> but might be easier to maintain by the kernel team
16:58 <apw> cjwatson, that is likely possible though pretty hard, but we may well be getting rid of it in favour of a bigger backport
16:58 <apw> cjwatson, wahts the issue?  isn't lbm-nouveau a dep of the x-server?
16:59 <cjwatson> yes, the issue is that that dependency pulls in grub-pc indirectly at a point when the installer is not prepared to install a boot loader
17:00 <cjwatson> (see scrollback)
17:00 <apw> cjwatson, so lbm is recommending grub ?
17:00 <cjwatson> no, linux-image recommends grub-pc
17:01 <cjwatson> and should continue to do so, that's desirable for other reasons
17:01 <apw> got ya
17:01 <cjwatson> indeed I think there's a sabdfl bug on the subject
17:01 <apw> hrm, tricky
17:01 <cjwatson> I'm trying some seed madness
17:01 <cjwatson> the kernel's already installed, so in theory I just need to get it out of the task
17:02 <cjwatson> actually there's another problem
17:02 <cjwatson> x-x-v-nouveau depends specifically on linux-backports-modules-nouveau-lucid-generic
17:02 <cjwatson> bryceh: ^- that will break if the user installed the PAE kernel
17:02 <apw> cjwatson, i believe that just got fixed ... like today
17:02 <bryceh> cjwatson, yeah fixed that yesterday
17:03 <cjwatson> ah
17:03 <apw>   * debian/control
17:03 <apw>     + Add -pae as fulfilling dependency
17:03 <apw>       (LP: #524792)
17:03 <cjwatson> ok, although I don't think the installer will be smart enough right now to pick the right now
17:03 <cjwatson> *right one
17:04 <cjwatson> we might have to arrange for the lbm package to be installed explicitly when installing the kernel, or something
17:06 <apw> cjwatson, damn ... i am not sure we'll be doing that for much longer ...
17:07 <cjwatson> doing which?
17:08 <apw> cjwatson, necessarily doing the backport that way
17:08 <cjwatson> right