No feedback.

The only possible explanation is, that anything is choking on the 
folders, most probably the c-client, so watch your web server logs.

Yes, that is exactly what I am saying.

Do you basically say that only the INBOX appears in the folder tree? 
Do more folders appear with different (smaller) accounts?

Also, there are three different Courier IMAP servers in use with 
different versions/build of both Gentoo world, and the specific build 
of courier imap/imap-ssl.  IMP exhibits the same behaviour regardless 
of which server is selected. The IMAP and folders work normally for 
IMP3, Thunderbird, Outlook, and Outlook Express, and manually 
telneting in to the imap port and navigating/listing the folder tree.

Well if you click on the expansion icon when in the 'Folder' view, it 
displays the Inbox, with an cut-off | showing the it intended to draw 
additional tree items, but displays nothing.



As far as logging, it doesn't generate any related errors in the log 
file, everything appears normal. There is no evidence of problems 
other than the lack of folder retrieval.

Addendum

=========

Even deleting the largest of the folders (the 272 MB one) does not 
allow IMP to retrieve the entire folder list.

And? What happens instead? Also, check your log files.

Running IMP on Gentoo 2005, connecting to courier-imapd.  When logging 
in to an account with a number of folders (20+) with a large number of 
emails in various folders (several folders have 500+ emails, one 
folder is 272 MB with 1300+ emails ), IMP never retrieves the folder 
list, regardless of the folder caching settings.