Had the same on mine... it's an O-ring failure. Depending on how much oil you've lost, you might have to have the level topped-up... which, I think, involves tipping the bike vertically?
Had same problem few weeks ago and, as LA rightly has said, probably blown oil seal (on sealed for life unit!) This will be covered by warrenty (if within period) There has been some talk about final drive problems but in my case fortunately it was simply a blown seal.
Lucky i wrote a letter of complaint to BMW UK asking for my warranty to be extended and year!! (It's an 04 12GS)
In a wierd way I'm sort of hoping other things will break, so the vulnerable parts will all have been replaced before my Africa trip in Sept 07.
Why do i love this bike so much I still cant think of a genuine replacement despite the occassional agro....
BMW certainly have not made any money out of manufacturing my bike. The irony is, that it's lucky it's a BMW as the dealship I use (Vines of Guildford) is pretty faultless with these issues once you manage to get hold of them...