As far as I'm aware from reading threads on this in the past, the fastmail WAP interface isn't actually managed/designed by the team themselves, and given the growing number of phones that are now supporting full web access, they don't see the point in investing time and money in a project that may be redundant fairly soon.
I keep meaning to try out Mail4Me which is a Java based mail client usnig pop3/smpt/IMAP protocalls. see
http://mail4me.enhydra.org/