Jeremy Arkes and Jose Martinez have an article in the latest issue of the Journal of Quantitative Analysis in Sports, purporting to show evidence for momentum in the National Basketball Association. Access to JQAS articles requires a subscription, but guest privileges to look at an individual article are available.
Using data from three recent seasons, the authors find "greater success in the past few games leads to a higher probability of winning the next game" (p. 13). Key to these results are statistical controls for focal teams' and opponents' long-term strength or ability levels (excluding the recent games), home/away status for a given team, and teams' number of days' rest between games. Some of the measures appear conceptually similar to an RPI ranking system, which accounts for teams' strength of schedule.
The study uses fairly complex econometric modeling and presents extensive results in tables. However, the authors distill the findings into easily graspable descriptions. For example, for each additional win a team has in its last 5 games, its probability of winning the next game goes up by roughly 2 to 4 percentage points.
I'm not sure, however, that these findings fit what the average fan would think of as "momentum." To some, momentum would suggest looking at teams that have won 5 in a row (or lost 5 in a row) and seeing how they do in their next game. Saying that a team with 1 win (vs. 0) or 5 wins (vs. 4) in its past 5 games has an increased probability of winning its next game (controlling for all of the aforementioned factors) is much more incremental in nature.