Not a good idea. Someone I know implemented Cisco WAAS and is experiencing performance problems. Specifically the CPU on the WAE device is spiked to 100% utilized. They are attempting to per-position 80GB of data including large 1-2GB PSTs to the WAE (Wide Area Application Engine). In my opinion this is a recipe for disaster. First off Microsoft does not support PST access via network shares, and once you read why you will understand why caching PSTs is an even worse idea.

http://logs.technet.com/askperf/archive/2007/01/21/network-stored-pst-files-don-t-do-it.aspx

http://support.microsoft.com/kb/297019

I ran my own little test by moving a PST to a share, starting wireshark (ethereal) and watching the activity. Accessing the PST over the network makes CIFS alone look very efficient.