*** Update on 07/11/2009 ***
The issue below has been marked as fixed. Refer to the release notes of 3.0.2 for a note on what has been fixed. I think now you can happily move back to VirtualBox 3.0 :-)
After installing the VirtualBox 3.0, I ran my Ubuntu under that. When I ran update manager, my VM kept freezing. I realized that not only with update manager, even with Firefox trying to download a huge file, the VirtualBox freezes. Looks like this a known issue and there is a bug artifact for this one. As of now, there is no fix available and there is no ETA as well when the fix will be available.
So I have happily reverted back to VirtualBox 2.2.4 version. I don't see that issue anymore.