I just noticed the problem for the first time. I recently upgraded my Ubuntu host to 8.04 (kernel 2.6.24-17-generic), and VirtualBox to 1.6.0. In the guest I installed the guest additions 1.6.0.
If the keyboard capture is on (green arrow in status bar) the dead keys don't work. Not by themselves (e.g. to type a quote), and not combined with a letter (e.g. to type an ë).
If the keyboard capture is of (black arrow in status bar), and the guest windows has the focus, the dead keys do work (in a Windows guest that is, in an Ubuntu guest they still don't work).
Keyboard layout is US International (with dead keys).
VirtualBox versions 1.6.2 and 1.6.4 have the same problem. When are all these keyboard issues finally going to be fixed?
This will drive me back to VMware server, once 2.0 is final (and I can use USB 2.0 in the guests).