Guilty,
All the ones sold in stores now are in fact the faster ones. I suppose it is possible if you buy from some store that still had old stock you could get an older one, but I doubt it.
I personally picked up a Microsoft Intellimouse Optical, very similar to the explorer, from Fry's Electronics. Look on the box for a sticker or logo that says "Now 33% faster." All the ones on the shelf I saw said that. It indicates that it uses the newer optical technology.
The ones that have the newer optical technology are great. Compared to the older models which had significant problems tracking when the mouse was moved quickly, these do a much better job.
Good luck