>Might the real problem at the moment (availability of pay-per-use web services) be that MS hasn't yet delivered the metering/charging mechanism(s) to enable people to do so.
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>Might it be that, for MS to do so any time soon, they would have to tip their hand regarding their plans in more concrete terms?. . .In other words, if MS plans itself to be the arbitor (middleman) between service buyer/seller, saying so now is too dangerous to their plans.

Not sure. It seems though that you could implement a monthly or fee-per-use system quite easily without MS Passport.

The UT has public webservice available for a yearly fee. This is rolled into the PUTM.