If you are looking for a log file for the App-V 5.0 beta you have probably gone through the Admin Guide to no avail. Thankfully they are using event logging now. For some reason this is listed under the sequencer section of the Admin Guide but not the client.
In case you haven't seen it the 5.0 beta is live.
Update on 2012-04-05 17:57 by Paul Fulbright
An interesting change...does this mean since they plan to EOL XP soon they are officially dropping support for it in App-V? Wouldn't be surprising. Also like that "any OS" is now the default.
Update on 2012-04-05 18:06 by Paul Fulbright
And quite possibly the nicest addition to the sequencer so far...
