It's still as of this evening pushing IMAP email immediately. Fusemail
does support IDLE so I suppose the change someone noted earlier in the thread might be the cause.
They're rather inexpensive for extremely reliable IMAP email provider ($1 a month per mailbox with a gigabyte of storage).
My school email (ucsb.edu) which is almost IMAP has the old five to fifteen minute delay, so I'm assuming it's something in the software RIM BIS uses and that Fusemail or other providers have enabled.