Thanks for the suggestions, but it doesn't seem to be a kernel or libcurl issue. I restored the original nokia kernel and that made no difference, and I tried downgrading libcurl since I already had the latest and that did nothing either

I don't think it is a problem with the n900 talking to the skype network, since conversations still work ok and show if the user is online/away/offline no problem. It is just the address book which doesn't seem to be talking to the skype account for some reason - which is strange since I didn't install any updates at the time the problem started.