back in 2006ish I used bluetooth proximity of my Mac running 10.4 and a Sony Ericsson cellphone to auto-lock my machine. It'd kick in around 10 metres away, which was about the size of my team's area. Quite handy, but the feature was removed.
Having bluetooth enabled on your cellphone might ultimately be the method used to crack in to your network though .. so there's a tradeoff of security in such a scenario.
