I'm a sole prop. And yes, I have an EIN. (I don't take insurance but I give patients superbills to submit to their insurers. You need to include a tax ID on those. [Which, without an EIN would be your SSN!]) Here's the IRS page about small businesses, EINs, etc.:
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Starting-a-Business
I went the sole prop route as I don't have any employees (otherwise I'd have started an LLC). I never looked into this for sure (maybe someone here could confirm for me?) but I had the impression if I were an LLC I'd have to put LLC everywhere (business cards, signage, etc.) I really didn't like the "vibe" that I felt "Dr. Still Kickin, MD, LLC" would convey. (Too business-ey / money-focused, rather than relationship / patient-care focused...)