Can anyone explain the exact registry keys required to get Udl connection 
pooling working.

I'm looking into the use of udl files for ease of maintenance, but without 
impacting performance.

Microsoft suggest udl files in conjunction with udl pooling, but don't 
fully describe the registry keys - I can interpret their description in at 
least 3 different ways (see below):

For greater performance, you can enable .udl pooling by adding "Udl 
Pooling" to the registry key "SOFTWARE\Microsoft\DataAccess" under 
HKEY_LOCAL_MACHINE. A Cache Size value greater than 0 (Cache Size=1) will 
enable the feature. A value of 100 is recommended

Thanks