In the spirit of this question...
Is there an indicator applet that tells me when I've received an email through any IMAP service? If so, where can I get it?
Use CloudSN (Cloud Services Notification). It supports gmail, google reader, pop3, imap, twitter, identi.ca
http://chuchiperriman.github.com/cloud-services-notifications/ (PPA available)
The best part is that it supports multiple accounts and notify-osd.
It is not a real gnome applet per se, but you can use gnubiff, it sits near the clock, in the notification/indicator applet: