@brickster, you and I have the exact same issue. Mine is also not switching from 3g to wifi automatically. With ch1naski chiming in, perhaps we have an identifiable issue.
+1, but is it supposed to hold a dl? I'm just used to it lol. For me 3G dl to wifi holds, sure it takes like 15-20 secs to "grab" 3g and resume but it works that way, just not wifi to 3g. IMHO not really an "issue" but maybe there's a fix?