This code is too dangerous. It is extremely unsafe in multithread applications and in applications that call this method too frequently (more that once in 1 ms).

Here's a nice one too:
http://www.javaexchange.com/aboutRandomGUID.html

That's not strictly speaking a UID though, and it could be kinda dangerous to suggest it is...

a) It can be called twice during the same millisecond
b) It can be called twice simultaneously on multi-processor machines

Something like this would be more secure imho