Support for Ultra High Speed MicroSD cards
Will those cards work with my device?


Yes, I believe SDHC UHS-I (Ultra High Speed) cards are backwards compatible and will "work" in an SDHC only device (that the Motorola Milestone appears to be).


  Do they need any special support from the OS?


Yes, in order to operate at the advertised higher speeds. "Use of UHS requires that the host device command the card to drop from 3.3-volt to 1.8-volt operation and select the 4-bit transfer mode." Wikipedia - Secure Digital


  Will I benefit from extra speed?


No, it will work like an ordinary SDHC Class 10 card.