There's for example Control system for a skateboard type device by Samuel Shiber for Saroy Engineering of Illinois, filed 1975:
Combination cycle seat-skateboard by Charles Persons II and Robb Harst, both of Ohio, for Persons-Majestic Manufacturing Company of Massachusetts, filed 1978, the idea being that the seat part of the bicycle can be detached to form a skateboard (see the tiny wheels underneath the saddle):
Radio controlled skateboard with robot by Steven Derrah of Rhode Island, filed 1997:
Solar skateboard by Chien-Ching Su of Taiwan, filed 2006:
Motorized skateboard by James Hawkins of California, filed 2009:
Self-propelled skateboard by Craig Gager of London, UK, filed 2010:
Finally, only published a few weeks ago, is Electric skateboard by Aaron King for Redrock Boardshop, both of Florida, with a very cool looking drawing showing it in action. Looks like he's on his way to work, though actually he has batteries in his backpack (which would be heavy, I think ?).