I am not sure if anyone is still interested in or if the solution has been found already but I've had some problems making this work and while trying to find a solution I've come across this thread.

This is my 0.02. I do believe this problem is Gadu Gadu specific as Yahoo messenger, which is also an additional plug-in enabled through pingin, works fine as far as I am concerned. I have no idea why exactly this thing doesn't work properly but it looks to me that N900 doesn't pull the contact list from GaduGadu servers when you log in so it simply cannot automatically update the contacts stored in the phone. If you enable any other protocol (I tried Skype, Google and Yahoo), after logging in all the contacts are added to the contact list which doesn't happen in GG's case.