What exactly do you want to control? N900 is not exposing itself through PTP protocol as a camera so I fail to see how GPhoto2 would apply here.

Actual Camera-related high-level APIs are open, you can drive camera yourself through GST Photography interfaces directly or through GDigicam library that provides a convenience level on top of gstreamer for application writers. Nokia's camera application is written with help of gdigicam itself.

So if you absolutely needed to control camera more controls are in place than what GPhoto2 would give you. If someone would implement PTP/IP server that drives camera through gdigicam, I'd be more than happy to use it too.

GDigicam's source code is available on maemo.gitorious.org for inspection and hopefully contributions.