I have been having problems getting the web proxy clients to work correctly through ISA. They work as firewall clients but not as web proxy clients. I kept getting site is forbidden for http sites even though it would work for https sites. Ping worked but POP3 did not. I tried many things but the got it to work as expected by changing the http redirector to not route the firewall connections to the web proxy(default setting) but to send them directly to the website. It doesn't make sense but I have to move on.
From my analysis of the event logs the install looks pretty clean.