Maemo is the software platform the Nokia 770 runs on. It is based on Debian and gtk with lots of great modifications for a handheld device. Nokia just recently released a beta version of its new software on maemo.org. There are a great number of detailed reviews already written.
There are screenshots in the above links for people looking at seeing the apps in action.
I am very pleased to see how well integrated presence/IM is into the existing functionality of the n770. For starters, I think the presence interface in the menu was a stroke of genius. It has an easy factor, which will make signing in not a chore. Though, I would like to be able to set available ("On") messages though in addition to Away status messages.
I also like the integration of contact management into the side menu.
Having been using the IM client (haven't tried voice yet) for the past day I would have to say that the following are my most desired features.
Although maemo integrates all my jabber contacts into my contacts database it does not populate that data with vCard information, which would be amazing. Instead I have to manually type people's names into the contact window. When you have hundreds of contacts this becomes quite a burden. Additionally, the left hand column in the chat view should contain all contact details for that user, their status message and other pertinent information.