Thanks for the shout out Matt!
Katie,
You should do something like what Matt suggests. Marketo doesn't have anything pre-built. You get to decide how and when things happen. For your situation, make the following:
Program: "newsletter sign up"
Form: Newsletter Sign Up with Email Address, hidden field=subscribe=true, button=sign me up
Flow: Fills Out Form: Change Status=Converted and whatever else you want.
Yes, you can make a page for this and/or use the iframe. BUT you can also go to the Form and get the embed code. Take that code and place it on your site. If you have an html coder, you can have them help you make it prettier.