Campaign Monitor has a great post on their Weblog, testing and discussing the actual state of Flash delivery in Email Campaigns.

Flash support across major email environments
PC	Displayed by default	Alternate content displayed	Security warning
Outlook 2003
Outlook Express
Thunderbird
Eudora 7
AOL 9
Lotus Notes
Mac
Mac Mail		Doesn’t matter
Eudora 6
Entourage
Web based
Gmail
Hotmail
Yahoo! Mail
Windows Live Mail (beta)

The results really do speak for themselves. Flash support across the most popular email environments is absolutely dismal. In the past, we’ve said that unless you specifically know the environment your recipients are opening your email in, avoid flash. Well, now we can revise that down to one lonely email client.

Flash was built for the browser, and based on these results – that’s where it should stay.