I don't know if Inventor can do that alone, but in my experience, a separate translating program is very useful. There are a lot of them out there, but the one that I have been using a lot is called Transmagic (http://www.transmagic.com). If you go to their website, I am pretty sure you can download a trial version, and can use that to translate your files into .stl with no problem. Its a pretty cool program because it also lets you choose how you want to translate [such as grouping certain parts together as one part, or a whole assembly together as a large part]. The only thing is that it doesn't recognize inventor .ipt/.iam's so you have to save them as something else first [i usually do .sat because its quicker to open than .stp]

Hope this helps,

George