I don't know of a direct way to HotSync with a web-based e-mail service, such as Yahoo or HotMail. However, you can have your web-based e-mail forwarded to your PC-based e-mail (e.g. Outlook Express). The actual forwarding occurs when you access your OE inbox. Note that this service *does* have a fee associated with it.
So, if you subscribe to the forwarding service, you'll receive your web-based e-mail on your Palm when you HotSync. Of course, you'll also receive the e-mail that was sent directly to your Outlook Express account.
That's the best solution that I'm aware of.
Hope that helps.