A couple of media outlets have questioned Rasmussen's generic ballot results, noting that it presently appears to be an outlier from other data in the RCP generic ballot average.

It might appear to be an outlier if you just glance at the data.  But if you dig a little deeper, you see something different. As at least one observer has pointed out, Rasmussen samples likely voters, rather than registered voters or adults.  Likely voter samples typically skew toward Republicans compared to "adult" or "registered voter" samples, and this effect may be especially pronounced at a time when Republicans are thought to be more energized than Democrats (and hence more likely to survive likely voter screens).  We can debate the pros and cons of imposing a likely voter screen a year before an election, but note that the other pollsters who have done so this year -- Democracy Corps (D), NPR, and Battleground -- have all shown either narrow Democratic leads or slight Republican leads.

But there's more going on here than just an A/RV/LV distinction, though.  If you go through the rest of the generic polls conducted this year, you'll see that almost all of the polling conducted this year (the relatively small amount not conducted by Rasmussen) shows a single-digit Democratic lead.  Why do ABC/WaPo and CBS show a healthier Democratic advantage?  To put it differently, Rasmussen is closer to Gallup's D+2 registered voter poll than ABC/WaPo or CBS.  Why is Rasmussen the outlier here, instead of these other pollsters?  And what accounts for the discrepency between ABC/WaPo-CBS and Gallup?

Good pollsters will generally ask their horse race questions toward the beginning of the poll.  This is because "policy preference" questions or questions involving other political figures can ultimately skew the result of a later horse race question, by unintentionally leading readers to view the horse race in a certain frame.  ABC/WaPo asks the horse race question as question 18.  CBS asks it after some thirty-odd questions.  And both pollsters ask the horse race question after a bunch of questions about a very particular policy topic: health care!  This is understandable since it's the hot issue of the day, but its also an issue where Democrats typically hold an advantage in the public mind.  This in turn probably left respondents somewhat more primed to answer "Democrat" than they would otherwise be.  Additionally, the questions about Obama (who is generally regarded somewhat more favorably than "generic Democrat") probably have some priming effects as well.

In other words, if you are asking which pollsters have it right, I'd probably put my money on Gallup-Rasmussen rather than ABC/WaPo-CBS.