While I wasn't paying attention, it appears that Digi International may have modified the naming on the Series 1 modules, they are now called "XBee 802.15.4" (note parenthetical "formerly Series 1" in their online store). The Series 2 modules are called "XBee ZB" but I've seen that for a while now.
I think that this was done because so many people assumed that Series 2 radios were better than Series 1, when in reality, the best series radio was the one that was best fit for the intended use - mesh network or point to point.
Renaming them to remove the Series designation is a good move, in my opinion, so that people are not simply comparing series numbers like version numbers, and assuming that higher numbers are better.
Indeed that was my initial assumption as well, so I guess it's a good move on Digi's part. But, the recent exercise in controlling the servo only reinforced my opinion that not only can the S2 do anything the S1 can, and just as well, at a lower price, but with only a minimum of configuration to boot. Hardly ever using transparent mode, that exercise led me to the minimal S2 configuration required to do one-way P2P transparent communication. Sounds like a good topic for a blog post or maybe a small series of posts. But basically there is only one step required. Load the coordinator firmware into one XBee, then use the coordinator as the receiving XBee.
This assumes that (a) The XBees come with the router firmware installed (a good assumption I think, at least it has been true of all the modules I've received), and (b) No other XBee networks are already operating nearby. The second assumption is probably good for someone working with their first pair of XBees, but if not, it only requires setting the PAN ID parameter in both units to something other than that of the pre-existing network(s). If one then wants to proceed to two-way P2P communication, then one additional step is needed, namely setting the DH and DL parameters in the coordinator to match the router's address (SH and SL).