Lots of people have been asking the never ending question of when PostGIS is going to get on the band wagon and support KNN GIST like other GIST based types trigrams, full text search etc. Well it's happened in PostGIS 2.0 and now committed. More of the gory details at Indexed Nearest Neighbour Search in PostGIS.
In short this will make point / point distance searches and rankings way way faster and help also with other distance searches by providing approximations to start with.
We are still preparing the PostgreSQL 9.1 2.0 32-bit windows builds that will have this functionality and should have that ready in the next day or so.
To summarize what you can expect. We spent a lot of time discussing and were torn between a box distance operator <#> and a centroid box distance operator <->, so we ended up having both. The reason being is that for some kinds of geometries e.g. streets that aren't diagonal a box distance operator seems to be a much better approximation of distance than a centroid box distance operator. For points of course the two concepts are the same and not an approximation so point / point distance you'd be better off using the new KNN sorting than ST_Distance + ST_DWithin as we have suggested in past. I'll be doing some benchmarks in the coming weeks comparing the old way and speed differences you can expect and perhaps throwing together box and centroid cocktails that combine the two weapons into thought provoking WTFs (or as Dave Fetter would say "That's very Rube Goldberg of you").