(this is all on 3g, not WiFi)
I have my Yahoo account set up on the phone. Right now, it works. Refresh...bam it refreshed.
A lot of the time it'll say that my username/password is incorrect. If I refresh, same thing.
I leave it alone for a few hours and sure enough, I'll get a little beep and it'll say that I have new Yahoo mail. I can refresh it during this time just fine.