The day pass for wifi is $10? I think by 2011 (date set to be completed), a lot of people will be on mobile data plans and it won't really be worth it to pay for internet on a 30 min ride.
Which line are you riding on? I take BART from Dublin/Pleasanton to New Montgomery every day and the only time I lose my signal is through the Lake Merrit tunnel and Transbay Tube.
I mainly commute between the city and Berkeley. The signal obviously goes dead in the transbay, but it's been pretty spotty in the above-ground parts, too.
