Just back from Florida and thought to give an update on this point. I think I raised it again recently in 2010, but cannot find the thread.
Used the Express card for a car hire booking last year (thro this site of course) and again they said the card had expired, but it did not seem to make much difference.
However (at MCO) there was no Dollar Express line but a sign to go straight to the pick up.
Was directed to the Thrifty desk ( same boothe, the dollar was busy) who were helpful but said that my card had expired and therefore they had no paperwork ready. However the chap (Scott) was VETRY helpful and said if I did not mind a weather would deal with the paperwork straight away. Which he did. (It was nothing to do with my credit card expiring)
In a discussion he said that the reason the Express card had expired was that it is done automatically if not used for a while. I assume that the previous 2 years of rental did not count as it 'went through' without the card.
I explained the situation that I use it regularly, and he said he would take a note of the Express number and renew it later.

Unfortunately a week later I had to take the car back (Ford Focus) as the low tire pressure light came on - seed my above post in 2007. I am aware that the light is not necessary an indicator of a problem This is annoying - Don't want to get stuck miles away from anywhere and preferred to take it back as was near MCO. Dollar very good and I could choose any car in the line, chose another Focus, paperwork done, started it up - same problem; light came up. Tried another Focus, same problem. Next Focus had scratches down the side. So chose a Chevrolet Cobalt. No problem. Used it throughout the rest of the holidays and a better car than the Focus ( even though it had tire pressure warning light; but no problem with it.)

Whilst doing this I mentioned the previous promise to renew the Express card and she showed me on the new rental agreement that it had been done.
SO next time I hope to be abl;e to by pass the MCO queue and go strighty to the parking lot.