There's no discounting the importance of momentum as a general rule, but this year has been a bit of an anomaly. Generally speaking, teams that finish the regular season poorly don't do well in the post season, although the Ravens this particular season are an exception to this rule. Perhaps the boost the team is receiving from the soon to be retired Ray Lewis is superseding the lack of momentum they carried from the end of the regular season. Typically, though, teams that finish the regular season the way the Ravens did don't excel. And let's not forget, it took a near miracle play and a brain fart from the Denver secondary to put Baltimore in the position to pull off this upset; their lack of momentum should have ended their season with a loss to the a Broncos last week.
And of course, the momentum which the 49ers will carry on the shoulders of their young quarterback may still, at the end of the day, be enough to stop the Ravens. Here's hoping.