I'm a bit worried about CPU/bandwidth explosión, nevertheless. And mobile bandwidth, that pay per byte.
Not that I have experience with mobile devices, but if you use zlib, the overhead of doing a ping should reduce to one byte plus a few bytes of padding every call in every direction. If you do a ping every minute, this bandwidth overhead is neglectable compared to the bandwidth overhead of presence updates alone. If you do polling and send/receive more than just the ping in one request, the impact of the extra bytes of padding diminishes as well. A while ago, someone claimed that zlib on mobile devices was not a good thing, but I'm not sure if there was proof of that.
cheers, Remko
