This one took me a little while to find, but there are two ways to modify IMAP logging in Exchange 2007. (BTW, I'm trying to get cross-site connectivity with IMAP to work.)
Open Regedit, go to HKLM\SYSTEM\CurrentControlSet\Services\MSExchange IMAP\Diagnostics and change the General value to something useful for you: 0 (Lowest), 1 (Low), 3 (Medium), 5 (High), and 7 (Expert).
You can also makes this happen via the EMS by using:
Set-EventLogLevel “MSExchange IMAP4\General” -Level High