On Natty and Oneiric non-Server command-line only systems (which can be installed from the Alternate and Minimal CD's), grub2 hands off to blank tty7 (as in bug 700686). As a consequence, a blank black (or purple) terminal is shown, and the system does not appear to be functional. This can be particularly confusing to novices, who sometimes assume that the system has not successfully booted. Switching to an active virtual console (for example, by pressing Alt+F1) yields a login screen and makes it possible to use the machine.
This also happens on a few Natty and Oneiric Server systems. It seems to happen on all such server systems running on VMware virtual machines and installed using the VMware Easy Install automatically installation facility (but it does not seem to occur with VMware when the OS is installed manually). The most obvious explanations specific to that feature do not seem to apply (see https://bugs.launchpad.net/ubuntu/+source/grub2/+bug/761830/comments/35). It would perhaps be worthwhile to investigate if the problem occurs on other kinds of unattended installations.
Please see bug 700686, bug 695658, and this bug's original description for some (incomplete) details about the cause of this behavior (and why it did not occur in previous releases). See original description and comments for details substantiating the above generalizations.