Sony has released the English-language version of their wireless update. We've installed it with Pocket Tunes 3.x, and it seems to dramatically improve streaming. We still occasionally get buffering, but it's much less often now.
http://ciscdb.sel.sony.com/cgi-bin/select-clie.pl
Select CLIE TH55 and then click "Drivers & Updates". Then select the wireless update and follow the installation instructions.