Dan, I have a hack for you that will work if you believe people will click into the form to enter their email address. It involves a little trickery, but a quick scan of the WP.Com TOS doesn't seem to disallow it.
Basically what you are going to do is create an image that *looks* like a signup form and link it to the hosted MailChimp signup form. An example of what I did can be seen here:
Step by step, I did that by:
- design your signup form on the MailChimp embed code tool. Since it's not really going to be all that usable, it's probably not worth including more than maybe a heading and the email address.
- While you're there, go to "edit forms" and get the link to your subscribe form.
- Once you have it looking good take a Snapshot of it (and possibly crop it out in an image editor as necessary). Note that you'll probably want to set a very small width.
- Upload that image to your WP.Com Media Gallery and copy that link
- Add a Text/HTML widget to your sidebar and simply create a link that has only that image as the content. Something like:
<a href="http://eepurl.com/bVDa" target="_blank"><img src="http://mailchimptest.files.wordpress.com/2009/06/signup.png" border="0"></a>
Now when folks click in there, it'll pop open your subscribe form. It's not the perfect solution, but it should work!