It's likely to have exactly the same kind of limitation, but SyncML? Funambol client (www.funambol.com) is available for the iPhone.

www.scheduleworld.com or my.funambol.com provide free "hosting".

So it's not a back up per se I guess.

Unfortunately this sort of problem has stopped me using Funambol. That and my new employer's somewhat dictatorship IT policy.