For information, modern libpcap and Wireshark versions can do some USB analysis. Depending on what analysis one want to perform, this can be enough.

Really, they can do it on any old nonspecific piece of USB hardware? That's quite impressive!

I don't know, I've hardly used it
But I have checked that it can definitely catch two-way communication with the hardware made by a proprietary driver and a device running a proprietary firmware, even if it runs e.g. in a VirtualBox Windows guest running under a Linux host.
Therefore, libpcap 1.1.x and later is potentially enough for some reverse-engineering purposes

I'm definitely going to try this out tomorrow! Did/do you or whoever wrote the SilverLink and DirectLink libraries for TiLP use this method to figure out the protocols?

Indeed, Romain used native Windows USB sniffer software extensively, for the SilverLink, the 84+ DirectLink, the 89T DirectLink and the older versions of the Nspire (CAS) OS.
Offhand, I don't remember the name of the software he used (but I know it's written in some traffic dumps committed into the TILP SVN repository), but it was not libpcap, as the capability of sniffing USB traffic was only recently added to libpcap.

I used libpcap through Wireshark (or any form of software USB sniffer, in fact) only once, several weeks ago, precisely to try out this capability of libpcap/Wireshark.

I used SnoopyPro to reverse-engineer the protocol used to communicate with the IM-Me's USB adaptor. It will occasionally stop logging (requiring a reboot of Windows) but the logs it generated proved to be invaluable.

I've now compared the logs and the sources of SnoopyPro and usbsnoop. This shows that Romain used the latter, at least for the 84+ USB logs.
For some Nspire logs, he might have used the newer http://www.pcausa.com/Utilities/UsbSnoop/ , I haven't checked.

For those looking for a hardware USB protocol analyzer, here's a list of some good ones:

http://comvcon.blogspot.com/2008/11/list-of-usb-protocol-analyzers.html
Open Logic Sniffer
Saleae Logic (thanks to benryves for the link!)
The Bus Pirate (thanks to benryves for the link!)

Also, here's a good Saleae Logic / USBee AX Pro compatible device for $65 (no analog channels) and $90 (one analog channel).

How does that $99 one compare with the full USBee AX Pro in terms of capabilities, capture granularity, and all that?

The $90 Logic-U Pro is exactly the same as the USBee AX Pro; the $65 Logic-U Plus is the same minus analog capturing.

That's awesome! Wouldn't that solve BrandonW's needs without costly reconstruction of the device, then, probably for the same price as it would cost for components to build those schematics?

It depends on what BrandonW needs; the USBee AX Pro has a high enough sampling rate to capture raw line states for low- and full-speed USB comms, but the software for the USBee only decodes "Setup,Data0,Data1,Ack,Nak,In,Out,Ep,Address, Standard Device Requests" packets, which may or may not be enough for his needs (for that he'd probably need a Beagle 480 or something with OTG support).

Makes sense, Ultimate Dev'r, thanks for clearing that up.