There’s a useful summary of recent work and thought here [link lost, sorry]. It turns out that *some* bones were found in the 18th century, but nothing very dramatic – nothing has been found since, apart from a few fragments which help suggest a date of around 2500 BC, and mediaeval and Roman finds.
John Barrett and Paul Devereux have both suggested its use as a platform, commenting that it would raise people to a level just about equal to that of surrounding hills, and apparently people standing on a missing Obelisk at Avebury would *just* have been able to see people standing on Silbury, as if they were actually at the same level. Interesting – though they must have had a very good reason to go to all that bother.
What I’d like is a map showing exactly where in the surrounding area Silbury is visible *from* – it’s largely hidden from south and east by natural hills. Oh for the time to do a circuit of it and make that map myself. Obviously what you can see from the hill itself would be interesting to tally, too. (Maybe a day off in July beckons…)