The upgrade to VBox 2.1.0 has some nice networking improvements (thanks!
I've verified that the "Shared Clipboard" is configured as "Bidirectional", and I've also upgraded the SUNWvboxguest to 2.1.0 on the guest, but this hasn't helped. I can confirm that the VBoxClient and VBoxService processes are running on the guest, and indeed other features (like full screen) are working fine.
In earlier versions of VBox I've had issues where cut & paste stopped working after a while, or became erratic, but a reboot of the guest usually fixed it.
Anyone else seeing this?