I have a bunch of domains that I've registered over the years, with plans for side projects that haven't yet been realized. Some of them are probably moderately valuable.
Over the years I've let some expire that I was sure I wasn't going to use, and those are all registered nowadays... but NONE of them are actually in use. Just squatted. That's not nice to see.
I've sold a few as well, for reasonable prices, once I was confident that the purchaser was going to use them -- e.g., wishmash.com. I've had offers for others where we couldn't work out a deal -- not because I was asking too much, but because I wanted to sort out a way to ensure the domain was actually used, not just resold, and that got too tricky.
I'd love a way to have a sort of long-term escrow for a domain name, so I could let someone use it for an actual project... and if they get some traction, I'll sell it automatically for a fairly low price, and if they abandon the project, the domain reverts back to me instead of going to squatters.