Clusty performs clustered searches of PubMed results (similar to the ClusterMed service that Vivisimo also provides, which also only clusters the first 100 results if you're not registerered). I'm still dubious of the value of this kind of limited keyword clustering (the results I've had from trial searches haven't seemed particularly illuminating), but it might be worth a try occasionally. Here's a link to the advance search form with the options pre-selected.