Install of Centos 6.0 to my Virtualbox had a couple hiccups.

First install did not have working network.

No Network

During OS install, watch for the easily overlooked button to configure networking. Click that and use Network Manager to configure networking to happen on boot, and check your IPv4 settings. Some say you could do this after install from the running OS, but it did nothing for me. I had to reinstall and use the Network button.
Virtualbox guest drivers

This forum thread shows how to get the Virtualbox drivers to compile/install.
Screen is still a small box inside my VB machine window after successful driver install

Click the VB Machine window (ie your host OS window gadgets) window size gadget. Even after a couple reboots of my virtual machine the Centos screen was a small box, not filling the entire available area. After resizing the host window, the Centos screen filled the entire available area.
Arrow keys do not work with using NX Client inside Centos

This is a keymapping issue between different X versions. Fixes I found using Google for Kubuntu 10.10 did not help at the time, so I found it easier to keep Centos 5.x around for this usage. My employer still uses old 4.6 Centos versions, and it's just easier to use an oldish local Linux to connect to that. I originally saw this problem with Kubuntu 10.04, and started using Centos 5.5 as my solution then. Yea, a whole OS install/machine for connecting to work seems a bit much, but oh well. NX client inside Windows Vista/Seven works OK too, but for some reason it seems better performance inside Linux, even if Linux is then inside Windows. Solution, use an oldish Centos as your NX Client .