This is an old post but repeated for the new people and those of us 
with less than stellar memories.

I've used UV leak detection about 5 times now and must say that it is 
very effective in pinpointing the exact location of the leak.  
Simply, a fluorescent dye tracer is added to the oil, the engine 
started, and using a black light (UV) the tracer-laced oil can be 
seen emerging from the offending structure.

It has impressed the VW dealer here in town and they now use it on 
their own irrascable leaks, calling me up to borrow my UV light.

A couple notes:  

the tracer-laced oil mixes quickly and comes out in 
a visible manner quickly.  usually in a matter of seconds.  it is 
impressive.  

also, it is inexpensive *if* one has a UV light.  tracer dye can be 
purchased at FLAPS for about $4 a small vial and 1/2 the vial is 
an effective dose.  so it works out to about $2 per test.

if you can't immediately pinpoint the leak and UV glowing oil is all 
over the place, one can start over by simply cleaning the offending 
area with carb cleaner and compressed air.  then restart the engine 
and watch again.  no need to re-dose the oil as the tracer will be 
good until the oil is dumped out.  or leaks out  :-)

UV lights range in price.  the best for this type of work is a 200 
watt focused spot.  these puppies run several hundred dollars.  
prices go down from there, the cheapest being a handheld unit that 
runs on AA batteries.  it costs about $30 and needs to be used close 
to the work and in a dim work area.

-dan