My guess is that ARPU is low because corporate clients are giving their employees a Nextel phone for PTT, plus a BlackBerry or other smartphone on another wireless carrier for everything else. Nextel PTT-only plans are cheap.
If Sprint takes PTT over CDMA and promotes the heck out of it, combined with 4G, they may not only be able to keep iDEN customers, but also transition them to smartphones, saving money for the company (one line, not two, to pay for) and increasing ARPU for themselves (though, of this ARPU, only $5 per month will be due to PTT, a radical departure from Nextel days).
As for PTT lineups, now that Sprint has stopped selling iDEN phones they're comparable to Verizon, with four PTT-capable handsets, one of which is a smartphone. AT&T will probably end up with more PTT-compatible phones in the short term, though I'd be surprised if they had sub-second call setup times like iDEN/SDC over non-1x.