The 736 is absent in the US, but dont look for DL Express to get it - pre 9/11, it was decided that the 73G would replace the 732s for Express but the order was never placed and DL Express was cut back in size. CO's 735 fleet is still very, very new, so I dont expect any 736s for CO in the near future either.
There was talk that, at some point, AA would look to the 736 or 73G to replace its huge F100 and MD80 fleets, but those decisions are also many years off, and have been further delayed by the events of 9/11.
So much has to do with timing, the 738 became the most popular member of the 737NG family for one simple reason, airlines were anxious to replace their 72S fleets and, size wise, the 738 was an ideal solution----about the same number of seats, increased range, and better economics.
As I said above, will airlines, like AA, need an aircraft like the 736 or will the regional carriers eventually, after lots of discussions with the unions, fly everything will 100 pax or less......once that question is answered, either the 736 will see new orders or it will quietly be dropped from the 737 line-up.