The manual for my newly purchased Toshiba 37XV365 LCD says "If the HDMI connection is capable of 1080p, and/or the TV is capable is refresh rates of greater than 60Hz, then a Category 2 HDMI cable will be required. Conventional HDMI/DVI cables may not work properly in this mode."

Given that the TV is 100Hz, refresh rate, do I need this cat 2 cable, or is this just sales talk, or are ordinary HDMI cables in fact, already cat 2 rated?

Sounds a load of rubbish - the 100Hz part (for waht it's worth) is entirely within the TV, and nothing to do with the HDMI lead.

A category 1 HDMI cable has not been tested for category 2 compliance, but that doesn't mean to say it wouldn't pass if tested. In practice, for short cable lengths, you're unlikely to see a problem.

But remember, it's digital, so it'll either work or not work. Don't fall for any of the marketing c**p that talks about richer colours and textures from a gold plated oxygen free blah-blah cable. You can't print better photos by spending £100 on a usb cable and HDMI follows the same principles!

The refresh rate they're quoting is on signals you're feeding in externally and nothing to do with the refresh rate of the display panel.

Sounds like it's for HDMI coming from a PC where you can have all sorts of refresh rates.

The important it from your instruction manual quote is 1080p

1080p requires roughly twice the amount of bandwidth of 1080i and a little bit over double compared to 720p. That's why cables are rated CAT1 (720p/1080i) and CAT2 High-Speed for 1080p.

Printers buffer information before output. This doesn't happen with HDMI signals.
Printers don't handle the continuous stream of high bandwidth data that displays are asked to cope with.

HDMI is more complex than a single string of 0's and 1's, so the "it's digital so it either works or doesn't" argument doesn't apply in the same way. Yes, there's a point where the data integrity is so compromised that the communication collapses totally, but before it reaches that stage there are levels of corruption to do with timing that can compromise signal quality without causing a total failure. [Please note: I am not talking about deeper blacks, better colours etc which is IMO complete marketing bollix]