I don't actually see the logic behind this device at all.
A woman who considers herself at risk of rape is presumably either in an abusive relationship, or actually in the process of being attacked. For the former, the problem is not how to catch/identify the rapist, it's convincing the woman to go through the terrifying and demeaning process of reporting and attempting to prosecute the rape. If she has been scared into silence by an abusive partner, she is unlikely to suddenly get the courage to use this device, for fear of retribution.
In the case of a one-off attack by someone she doesn't know/doesn't feel threatened by, I don't see where the opportunity to use it arises.
On a similar note, for anyone able to get to London (not many of you, I realise!), there is a Reclaim The Night March in central London next Friday (25/11/05). The march is women only, but there will be a mixed rally and party afterwards at the University of London Union in Malet Street. Andy, if you do read this, drop me an email at maevie at gmail and I'll email you the flyer, I'd really appreciate it if you could leave some around at uni.
<spe> maevie - proving dykes can't fly
<Dor> You're levelling up and gaining more polys!