Nate Silver has a really good post up today about the NY-26 special. Silver, I think, gets it right: it's important not to read too much into the results of any special election, but it is worth assessing the data and getting what we can out of it -- even while being very careful not to extrapolate too much.
To be more specific: what everyone wants to know about NY-26 is how Medicare plays. And, if the polling is good enough, it should be possible to get at least a bit of information about that. But Silver is also right that it's really easy to read way too much into those data (for example, as he points out, those who would have supported the Democrat anyway may say that it was because of Medicare because that's what she's talking about). And what Silver doesn't mention is that knowing how Medicare plays in one place in spring 2011 doesn't necessarily predict how it will play elsewhere in November 2012. It's information, and you always want to add information, but you also have to be careful not to pay more attention to the data you happen to have than to the (potentially far more important) data you don't happen to have.
Silver is also quite right that for questions such as "how is Medicare playing?" the actual W/L election result isn't important at all -- what's important is to get good estimates of how that issue affected the margin. On the other hand, as I said before, the result can matter if it changes expectations about fall 2012, because those expectations then affect important decisions by political actors. Although, again, be cautious: we're all going to pay a lot of attention to NY-26 right now, but we can't tell in advance how important it will be to expectations a few months down the road. For all we know, there might be some other special that goes the other way in a few months, and that one will be on the minds of on-the-fence politicians when they decide whether to run for Congress.