I too send out several thousand opt-in emails per week so I have to put my two cents in as well. If your mailing list is opt-in (may you fry in hell if it's not) then you should ask the subscriber whether they want to receive text or html format. Bottom-line, this is better than trying to get cute by using content-type mime/alternative primarily because the AOL/Hotmail/and Yahoo do a bad job of reading your headers (and if you look at your mailing list, you will find that AOL/Hotmail/and Yahoo domains account for a large percentage of your audience. Also, there is a virus going around right now in a message with a mime/alternative header, so I dump everything with that header right into the trash unopened.

Better yet, just send everything as text.

Lest you all berate me as a Luddite or amateur, I did build a fancy/schmancy mime/alternative mailer that applies some smarts to the message format based on the domain, but I sometimes get replies from people saying that they can't read my message and it is often due to the mimetype.

Also, I would be interested to hear about any difficulties other people have encountered in sending spam to hotmail accounts.
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