I'm in the same boat. I'm the author of http://wikis.sun.com/display/SunOnMac/UsingVMWare
but with reasonable MacOSX support it seems (seemed) like a good time to switch to VBox.
VMWare gives access to the host OS as a side effect of their NAT driver. This lets me set up a more or less private 1-1 NFS connection, which gives me the sharing I need without messing with (faulty) shared folder mechanisms. Coming to VBox I see that isn't part of either the NAT or the internal modes.
I might try to use the bridged mode, but the problem there is that every time my laptop's IP address changes (yes, I develop mobile) the guest/host NFS connection would have to be renegotiated. Seems like a loser to me.
Here's a plea (maybe I'll upgrade it to a bug or RFE too): Shouldn't a design which gives you a number of virtual machines running on with one host machine, also give a way to set up a virtual LAN among exactly those machines? If you gave me a bunch of physical machines in a room, but told me they could not be networked together, I'd be frustrated. In fact, I'm frustrated now, for virtually the same reason.
The NAT mode is default; that's a good choice, and the docs state why. (I find the VBox docs are unusually informative; kudos.) I think there should be an option (turned on by default!) which makes the host visible on the VM side of the NAT server, under a fixed IP address (and/or name). The converse should be done also; not sure how to do that short of making a new software network interface in MacOS. Since this is hard for the casual VM user to do, it should just work out of the box.
The point is that VirtualBox should allocate me a pair of private, stable addresses, for the guest and the host, and make them visible to each other, and to themselves, under those addresses.
Anything less than stable mutual visibility makes setting up NFS and SSH links too hard for a non-specialist like me. (I do Java VMs; I'm not a networking expert, nor should I have to be. I own a Mac so the system software can do networks for me.) By the way, I'd be overjoyed if there is a way to build such a virtual LAN but I missed the tutorial on how to set this up. I'm looking forward to adding the recipe, if or when it works, to http://wikis.sun.com/display/SunOnMac/UsingVirtualBox