The boards improved layout makes this shield compatible with more Arduinos, makes it easier to use and opens new possibilities. The main differences from rev.1xx are outlined below:
The SPI interface has been moved to ICSP header making shield compatible out of the box with big Arduinos Mega and 2560. Standard size boards, such as Duemilanove, UNO, as well as clones with classic connector layout, are also compatible with this revision of the shield. This compatibility is maintained on a software level as well all necessary code changes are performed using conditional compilation preprocessor directives.
USB connector has been moved to the opposite side of the board. All external connectors are now in the same place making enclosure layout easier.
MAX3421E RESET line is no longer handled by dedicated Arduino pin. While MAX_RESET pin is still used in current version of USB Host library, it will be dropped from the next major revision of the code.
The shield is compatible with official full-size Arduino boards (Duemilanove, UNO, Mega, Mega 2560, and possibly others), as well as clones which supply both 3.3V and 5V to the shield. One example of such clone is BlackWidow; if you are aware about other compatible clones, please let me know.
The shield is compatible with current version of USB library.