The question of the moment is what effect Scott Brown's victory will have on national politics.
It's important to note that his election to the Senate does relatively little to change the overall balance of power in the country. See, for instance, Joshua Tucker's helpful chart:
The loss of Democrats' filibuster-proof majority seems to eliminate the prospect of passing the health care bill through conference committee, but for other legislation, the shift of the pivotal voter from Ben Nelson to Olympia Snowe in the Senate is likely to have a relatively small direct effect. Nelson is currently paying a heavy political price in Nebraska for his support of the health care bill and is unlikely to take a similar risk on future legislation. (On a more technical level, Tucker also notes that the gap between Nelson and Snowe's ideal points is probably relatively small -- see, for instance, Simon Jackman's estimates [PDF].)
Similarly, we knew Democrats faced an unfavorable environment two weeks ago and that the health care reform plan was relatively unpopular in national polls. Not much has changed on either front.
What matters, however, is the collective interpretation of the election. Even though Brown's victory was an ambiguous amalgam of national and local factors, including Coakley's hapless campaign and poor economic conditions, the media is already portraying the outcome as a referendum on President Obama (though a majority of Massachusetts voters approve of his performance) and health care (even though Brown supports a very similar state-run plan in Massachusetts). Debatable as they may be, these interpretations may quickly become conventional wisdom -- indeed, many Democrats have already endorsed them.
The most relevant comparison to the current situation might be electoral mandates. The seminal political science research on the subject shows that opposition party legislators tend to deviate from their typical voting patterns in the direction of a perceived mandate for some period of time before returning to normal.
Given the Democratic tendency to panic in these types of situations, we may see a similar shift in voting patterns or a change in the party's legislative agenda. Pundits will likely claim that Democrats should yield to public opinion as expressed by Massachusetts voters. But it's not at all clear that such moves will prevent significant losses in the November midterms, nor that there is a "message" from Brown's victory as such.
Update 1/20 1:50 PM: Based on Brown's voting record as a state legislator, political scientist Boris Shor estimates that he will become the Senate filibuster pivot rather than Snowe. As I've previously argued, I think Brown moved right to motivate the GOP base in a low-turnout special election, so I'm skeptical he'll pursue such a moderate course (at least right away). But if Shor is correct and Brown is between Nelson and Snowe, it reduces the rightward shift in the filibuster pivot, meaning that Brown's win would have an even smaller effect than we might have otherwise thought.