It depends on which phone you have.
I have a SE t610 and I cannot get irda dialup to work for gprs or gsm. It works fine for sending/receiving ringtones/pics/java games etc.
This is on the c760, I can get it to work on my old 5500, using an early rom 2.31 AFAICR.
It works fine with bluetooth on the c760, which is much better than irda anyway, so I am not complaining.
Peter.