I've finished much of the work on improving the in-tree USB stack, apart from adding device IDs. This is just the initial round. Now that all the cruft has been removed from the in-tree USB stack, we'll be able to support it better in RELENG_7. I plan on merging most of the client drivers into RELENG_6 after a couple of weeks, depending on how testing goes. So far, so good. I've added about 30 new devices. In fact, I've added so many that I've had no time to update the manual pages. If there are any doc people that can help synchronize the man pages to what we now support, that would be great.
After removing the obfuscating macros from the code, it is clear to me that adding some primitive locking to the USB stack might be possible now. It isn't going to happen for 7.0, however.