You can't. By definition, SMS is for "text" messages only that are less than 140 characters (some carriers allow up to 160)... so adding anything that is NOT "text" causes it to be sent as a MMS.
You can, however, send it by MMS or e-mail it by scrolling down and clicking "Share Contact".
I suspect that when iOS 5 rolls out, you will also be able to share via the iMessage app...