I recently installed Ubuntu 10.10 on a computer that was previously running 10.04 (that worked fine).
Now, each time I boot up, it starts up in a command line login prompt. I can login and it stays at the command line (as expected). I can then manually start gdm with sudo start gdm and it works fine. I can also enable compiz (using proprietary nvidia drivers) so I'm reasonably confident that it's not a driver problem (at least not in the sense that the drivers just flat out aren't working).
Interestingly, if I leave it at the command prompt without logging in, after about 5 or 10 minutes, gnome starts up on its own. I'm not sure what is causing this.
This is what dmesg | tail gives me after a manual start of gdm:
[ 15.664166] NVRM: loading NVIDIA UNIX x86_64 Kernel Module 270.18 Tue Jan 18 21:46:26 PST 2011 [ 15.991304] type=1400 audit(1297543976.953:11): apparmor="STATUS" operation="profile_load" name="/usr/share/gdm/guest-session/Xsession" pid=990 comm="apparmor_parser" [ 16.606986] eth0: link up, 100Mbps, full-duplex, lpa 0xCDE1 [ 18.798506] EXT4-fs (sda1): re-mounted. Opts: errors=remount-ro,commit=0 [ 26.740010] eth0: no IPv6 routers present [ 90.444593] EXT4-fs (sda1): re-mounted. Opts: errors=remount-ro,commit=0 [ 189.252208] audit_printk_skb: 21 callbacks suppressed [ 189.252213] type=1400 audit(1297544150.218:19): apparmor="STATUS" operation="profile_replace" name="/usr/lib/cups/backend/cups-pdf" pid=1876 comm="apparmor_parser" [ 189.252584] type=1400 audit(1297544150.218:20): apparmor="STATUS" operation="profile_replace" name="/usr/sbin/cupsd" pid=1876 comm="apparmor_parser" [ 351.159585] lo: Disabled Privacy Extensions