tried many different configs with N900 but no luck

the videocall window appear, the red led of the front camera is on for a second and then it close and appear a message in the conversation window:

error with your webcam
conference error
the call has been terminated

the audio call also stays on calling, u can adjust volume, but no sounds and usually crash when the other answer the call

no usefull messages from terminal
just "floating point exception" when it crash

Just to know , i tried also Pidgin on Ubuntu, and audio/video call work if the other person is connected via Gmail website. (i made a Gtalk test) Also Empathy is working with videocall on gtalk.

With the default IM client on N900 (telepathy) videocall start fine if you are the caller, they can see you but you cannot see them (sometime is green screen). if you receive the videocall, when you answer you dont see their video, and the caller never know that you answered.

all this tests are made from N900 to PC with Gtalk from website

i dont know from N900 to other phone, for example Android with Gtalk

but from pidgin or empathy on ubuntu is not possible to videocall people with gtalk in Android...