I think CX747 is right. I read in Airways that Boeing is now phasing out the 300/400/500 production line (the 200 went out of production 10 years ago), what I think is happening now is that Boeing still builds the 300/400/500 series but doesn't take anymore orders. I don't know who still has those on order but now that there are the 600 thru 900 models it is unlikely that an airline decides to order from Boeing an older 737 (I think Alaska ordered a few 400's not too long ago but they must have received them all by now).
The last 737- 300/400/500 will probably roll out of the Boeing plant in 2002 at the latest. You can be sure that in 2003, the 300/400/500 will be out of production.