It seems all the major GPS apps (BB Maps, Google Maps, Orange Sat Nav) use GPRS only- or at least I haven't been able to get them to work with Wifi. I did find a navigation app that seems to work with Wifi - TrackMyJourney. ( TrackMyJourney
Push email will also work through Wifi.
I think that because the 8820 is relatively new very few applications take advantage of Wifi. Hopefully this will change over time.