Clearly, some of you are encountering legitimate issues (recording should NOT stop after 12 seconds or whatever). Most of you (and I lump myself into this latter group) are, however, probably encountering an apparently intentional limitation of the hardware.
To address the memory card, per page 3 of the 70D's user manual, the required read/write speed is 20MB / second; anything with an equivalent or faster write speed should work just fine, and I can say with confidence that 30MB / second cards from Lexar and SanDisk work without issue. As noted by several posters here, manufacturers play pretty fast and loose with their specifications - the trick is to find a card with a 20MB / second write speed, which can be significantly harder than it should be. Using Lexar as an example, their own web site says that their 100x, 200x, and 400x cards will read at 15MB/sec, 30MB/sec, and 60MB/sec (respectively) - and that "write speeds will be lower" (they do not say how much lower). You have to move to their 600x cards to get guaranteed minimum sustained write speed (of 90MB / second).
Unfortunately, even if you have a card that's up to the task of video recording, you still have another hurdle to clear. The camera itself is officially limited to just 29 minutes and 59 seconds per clip. This is reflected in the camera's own menu and in the user manual on page 266. That said, however, my own experience has been that it's only the first clip which is limited to 29:59 and that subsequent clips in the same session (no still captures, no switching away from movie mode), though split into 4GB chunks, will only be limited by the card's and/or the battery's capacity. I guess I should be thankful, but I find this inconsistency to be frustrating.
Hopefully this saves some of you some money and heartache.
-j
