Lawprof David McGowan opines on this at the Legal Ethics Forum. The post is long (because the issue and the relevant law are complex) and interesting, but here's the bottom line:

Did Judge Alito violate any rules by sitting on the case? Deborah Rhode opined on NPR that Alito's sitting on the case was "a violation of judicial ethics 101." Steve Lubet was more forgiving, describing it as a mistake justifying an "oops" response, but not an episode calling Alito's ethics into question. Steve Gillers pretty much agreed . . . .

I think Professors Lubet and Gillers are right, and that Professor Rhode is not. One can approach this question in a pragmatic, purposive manner, in which case it is clear that Judge Alito did nothing seriously wrong, and that this episode provides no basis to question his ethics, or in a strictly literal, formalist, manner, in which case a case might be made that he violated rules against conflicts by hearing the case the first time but then properly recused himself when a complaint was made. That case has not yet been established, however, though the reports cited above imply that it has.