I attended a presentation by Daden, who are doing a lot of very impressive and interesting things in Second Life (and other virtual worlds). I thoroughly recommend having a look at their space in Second Life, where they have some great mash-ups with Google Earth. I would post some links here, but the ones I could find on their site aren't working, which is a bit rubbish.
Aside from the things appealing to my inner geek, there were also some ver interesting applications in learning. One project I found particularly interesting was the JISC funded 'Problem based learning in Second Life'. We were shown a simulation of a road traffic accident used for assessment. The detail was quite incredible (including the ability to listen to the patients breathing, which adjusted over time based on the actions of the attending paramedic). The medical sciences as usual are front runners in the use of new technologies, but I could see many applications in other domains.
The question that does still bug me is whether we should be doing this in open worlds, like Second Life, or if we should be using more private spaces. Perhaps a happy medium will be found in the Second Life Grid, which seems to be looking to offer the best of both worlds... so to speak.