In American political folklore, bipartisan bills are virtuous, wholesome, and god fearing. The rationale being that if both parties agree that the bill is a Good Thing, then it must be OK (or at least non-toxic).
Needless to say, both parties want their bills to gain the label of "bipartisan" and they accuse the other party of "obstructionism" if they fail to jump on board. Once the minority party has voted for a bill, they cannot campaign against it. The Obamacare bill, currently raising political blood pressure is a fine example.
The real question for the minority party is whether a majority party bill is of sufficient worth to support. The working politician firmly believes in "scratch my back and I will scratch yours." Unless the bill is really really bad, the minority party is tempted to offer "bi partisan" support, in return for a promise of earmarks, committee assignments, and support for it's pet projects.
Trouble with the "go along and get along" strategy comes at election time. The voters have great difficulty seeing much difference between the two parties, especially when they have been "bi-partisan" all the time.
So far this administration, the Republicans have been good about opposing things that smell really bad. They voted against the Porkulus, Cap and Tax, and Obamacare. They need to keep it up, any weakening will cost them in 2010.