I realize that given the stringent circumstances required to reproduce this problem (a law student using Westlaw through OmniWeb 5 on Mac OS X), there is a very good chance that the following will help no one, but I'm sick of doing research at the moment and felt like sharing. As I mentioned in a previous entry, OmniWeb 5 is the best browser I've ever used on any platform. Ever. As such, I'd obviously like to use it for researching my current law school project through Westlaw and LexisNexis (i.e., not be forced to use another browser just for these tasks).
To get Westlaw working, you need to remove all of the Westlaw cookies and then set your USER-AGENT (through Site Preferences) to either "Netscape 6.2 (Mac OS X)" or "Netscape 7.0 (Mac OS X)" — these are the only strings offered by OmniWeb that seem to give Westlaw what it wants. On a somewhat related note, the "Safari 1.0 (v85)" string does not work, though Safari itself works fine (but is not 'supported').