I think they use something very simmilar to the Jabber protocol. Mxit (server) quite happily connects to other Jabber servers, and you can see/talk to people with a normal Jabber client with ease. I tried decompiling one of the applets (to see if I could get it to work on my phone), but the code is obfuscated, and damn hard to follow. Using the Nokia emulator, you can check the network traffic generated by the app, and it looks nothing like the Jabber Protocol (I think I may have just contridicted myself...).
In the end though, I found mobber (
http://gryf.info/mobber/en). It works on
all phones (that allow full wap access, which my phone doesnt), and is based on the standard Jabber protocol. And best of all, it is open source. Initially I was confused as to why the MXit guys would develop their own protocol for this, when Jabber would work fine, then afterwards it occured to me it might have to do with the XML part of Jabber incuring more overhead, wich means more bytes sent, which means you spend more money. But after checking the size of a 30 character message sent with MXit, and a 30 character message sent with Mobber (normal jabber client), it turns out standard Jabber ends up being smaller. But only because it doesnt get a 9 byte confirmation message back from the server...
Anyway, bottom line is it looks like Mxit use their own protocol based on the Jabber one. Mxit guys can talk to Jabber guys, and vice versa. If your phone does not support Mxit, use mobber (you will need to create a Jabber account beforehand though).
Of course I could be horribly wrong....