I am quite impressed by the last release of VirtualBox, the 3.0.x, it runs fast using low resources. Having Ubuntu 9.04 as guest on Mac OS X goes smooth, the auto-detected focus makes usage of keyboard and mouse straightforward.
The only problem I encountered is using bridged network mode with airport. I get IP address via dhcp, setting for dns, but the gateway is not reachable. The other computers in the network are visible via IP, but access to internet doesn't work.
I need this mode so other nodes can connect to guest OS on my laptop -- here falls the SIP phones I used to test the development of Kamailio (OpenSER) and SIP-Router.org. Internet access is nice to have although I can do it from the host OS, having it on guest OS makes everything move faster.
The solution I found is to add a second virtual network card to guest OS which is set in NAT, and voila! internet access. Nice!
Don't forget to install guest additions, they are marvelous for Ubuntu 9.04 - full screen in the way you barely notice you haven't booted directly Ubuntu.