Nice idea but needs more thinking.
Tommy

There would definitely be some liability involved in letting someone else borrow your vehicle. You'd have to hire a lawyer to work out the details and put together a mountain of disclaimers. In addition, you'd have to be extremely competitive with pricing or else people will just go with U-haul. How much money do you think someone would want to loan their van/truck to you? Probably more than U-haul charges.

what will make your service cheap?

The idea is nice, but I think your target audience is probably not very deep-pocketed so I wonder whether this well ever generate enough revenue.

I don't know... with the right sort of networking/infrastructure, I think this could work. But I think the real idea would be the networking infrastructure/payment system, which isn't really covered by this idea. There are lots of things that would be made better by such a system (carpooling, grocery shopping, book lending, anything that people help each other out with and could help strangers out with). But when do we get the system?

In the UK at least, there are plenty of man-with-van type businesses, perhaps you could build a portal to promote them. Charge the businesses a fee for inclusion in the directory, allow advanced listings (like paying yellowpages for a bigger ad).
A map mashup would allow you to display the locations of the businesses and possibly the coverage area along with different capacities etc.

I would be worried about theft and insurance... I think the insurance part will end this idea.

Theft would be my worry too... let me "borrow" your truck so I can move somethings (by move he means rob a house, use you vehicle to transport the stuff, and then take your truck too...)

People don't steal UHaul trucks very much (or do they?...) because they crappy vehicles. On a few moves I felt we would go faster with a Honda Civic pulling the truck.

You could make it work with the right spin.. I hope you are a marketing genius....