I have exactly the same problem. I have a Dell Precision T7500 pretty much out-of-the box, with a Quadro FX 580 and a pair of 1T drives. At one point I was dual booting Win 7 and Ubuntu 11.10 with no problems. Then I upgraded to 12.04, and problems made the system unusable. I had modules crashing with the appropriate dialog boxes, when I rebooted, I would get stuck at a command line login -- which didn't work. Eventually I gave up, wiped out the partition and reinstalled 12.04. After the normal problems trying to get my video card to work I realized that the system acted the same way it had before. I.e., very slow, processes crashing; unusable. I finally got what appeared to be a usable system by wiping out the partition, doing a fresh install, and going in to recovery mode on the first boot. From there I ran fsck, started the network, and installed nvidia-current and did an update/upgrade -- which was extensive. But the system seemed normal when it came up. I installed and configured autofs and all was well. Until I rebooted, then I was back to the same problem. I found that all I had to do was boot to recovery mode, start networking, then resume boot and all was well. I tried booting without going through recovery mode and waiting several minutes (like 15-20 or more) to get a usable login prompt, but could only log in as a local user. I did a sudo -i and ran initctl list and found that many of the processes were still waiting. I also had no IPv4 address, though I did have a pair of legal global IPv6 addresses. I ran dhclient -- which took a very long time to run, and had another probably unrelated process die in the interim -- and got an IPv4 address and the correct /etc/resolv.conf. Syslog shows that many of the processes started by dbus failed to activate.
A Google search shows that this problem is not unique to Ubuntu, but also affects at least Red Hat and Debian, and has been for a year or more. Clearly the upstart/dbus mechanism is not quite ready for prime time.