I'm also traveling to Denver this weekend (for some concerts at Dick's Sporting Goods park...) and have a (long-held) reservation to pick up on Friday.

I went online last night to see about getting a better price; one of the places I did some comparison shopping was Alamo. Despite their "last minute special" for Denver ($20/day for a standard) they had no cars. So I went over to National knowing that it was the same inventory and they would offer me a reservation.

Went back this morning and discovered that National now reports also being sold out.

I think their inventory is large enough that you should be able to find SOMETHING when you arrive -- at least I'm counting on it -- but then again, we might not have a ton of choices...