For those of you in the early July timeframe (mine starts July 6), I'm starting to see the expected rapid decrease in rates, at least at Alamo. (I also see Dollar has dropped, Budget not yet). In just the past 3 days, I've seen my 9 day rental drop from $410 total to about $220 for a standard, using Costco code and upgrade from midsize. The drops seem to be most dramatic for common car sizes, not for SUVs, vans, etc. The rates have even dropped multiple times within the day. Of course, I'm hoping they go down a little more, but I doubt I'll get too much lower.
Also, at least at Alamo, the current set of last minute specials went out to June 25, which was yesterday, which leads me to believe they'll be releasing some new last minute (code IG) specials soon. If I remember correctly, the last set of last minute specials for Alamo did not include MCO, but the rates using traditional codes were better than last minute rates anyway. I would also expect Budget to come up with a new "Wendie's special" pretty soon; they've had some real good deals with that.
For those of you in the later July timeframe, you may not be seeing your rates drop yet. Be patient; the trend lately has been good and I wouldn't expect it to change. (Do book something non-binding though!)
Finally, under no circumstances should you lock yourself into a "prepaid" rate, at least for a common type vehicle. Rates seem to be dropping rapidly. Just keep checking and rebooking, right up to the day you leave.