When you put a call on hold, call a second one and then want to connect the two and disconnect youself, you press "4 + send" or something like that. I'ts some kind of standard 3G net function...and it's used in the swedish operator Tre in their Switch service. Unfortunately, it doesn't work on the N900.
I had a chat with Nokia/Tre where they explained that this possibility is NOT implemented yet om the N900.
