I ran into an odd issue the other day where the Approval workflow for a Publishing site simply would not work.  .Net 3.0 was installed properly and the site was otherwise functioning perfectly so it didn't appear to be a problem with the install. 

 What was happening was that when I pulled up the first page in a workflow, I received one of these two messages:

"The form has been closed" - which is the lovely generic InfoPath version of the Sharepoint "An error occurred" message.  Not terribly useful

- or -

"The server is currently being taken down for maintenance. Check with your system administrator to see when this server is scheduled to come back online.

Click Try Again to attempt to load the form again. If this error persists, contact the support team for the Web site.

Click Close to exit this message."

... yet, the site was running fine aside from the workflow.

The Solution

Somehow the site had gotten itself into a partially quiesced state where some of the site was convinced that the whole farm was quiesced while other parts thought everything was running fine.  The fix was to go into the Quiesce Farm option in 

Central Administration > Operations > Quiesce Farm and then restart the farm.  After that, everything worked fine.

An interesting thing that pointed even more to this was the error in 12/LOGS that read: "Loading event log failed, because the form was quiesced".