Check out the Invader MK-III. There is a guy in Michigan who owns one, and I think he wants to sell it. Here is the link of the Yahoo site he set up espcially for the Mk-III. Aside from his research, there isn't much on the web.
As stated above, weight is the penalty.
You can have a jet engine, retractable gear, in-flight adjustable pitch prop, flaps, slats, .. etc.
Lots of things prohibited by LSA & other classifications are allowed under ultralight rules.
Basically, all you have to comply with are empty weight (max. 155 lb. un-powered / 254 lb. powered), airspeed (24-55 knots powered), and fuel capacity (5 gallon max.) restrictions... and operate only during daytime while staying out of congested areas / controlled airspace..
FAR 103 is great that way .. lotsa freedom !! :gig:
Because of the restriction in maximum cruising airspeed, you would be hard pressed to find a case where retract gear is justified on a powered aircraft. At 55 MPH the reduction in drag is not very significant.
Because of the weight, you would be hard pressed to find a case where you can afford the extra weight on a 155 pound glider.
One of the long-established tenets in glider design and soaring is that a well-faired fixed gear is much lighter and often lower drag than a mediocre retractable gear.