Thank you, but I only want to carry one USB cable. Period.

Zagg has been very responsive. They actually called me back today, New Year's Eve, within a couple hours of my first inquiry, to let me know that they had located a N900 and tested it with the ZAGGSparq and confirmed that the N900 appeared ill-behaved. They also found that the "Power saving mode" seemed to have some impact on if it would charge properly or not.

I am coming to the conclusion that this is basically a fault of the N900 -- I should be able to plug its supplied USB cable, or any "real" USB cable, into any reasonable USB power device out there (which the iPhoneiPod have kindly populated in cars and airports everywhere) and have the N900 charge.

That the N900 had problems even recognizing the supplied AC-10 suggests to me that they need to fix this in firmware. If Nokia can't, in my opinion, this is a significant enough problem to warrant a recall/replace program.