The Digital Rebel T2i offers HD video recording. To record movies the camera operates in Live View and makes a continuous recording of what is seen on the LCD screen as either 1920x1080 (1080p) HD video at 24, 25 or 30 frames/sec, 1280x720 (720p) HD video at 50 or 60 frames/sec in 640x480 pixel 4:3 SD video at 50 or 60 frames/sec. The T2i has a built in microphone on the front of the camera for recording mono sound plus the ability to record stereo sound from an external stereo microphone (standard 3.5mm jack).
The Digital Rebel T2i has unique video mode found on no other EOS DSLR which Canon call crop video. It only operates in SD VGA mode (640x480 pixels) and it takes the central 640x480 pixels of the sensor and records them, rather than the normal video mode where the whole sensor (or a least a 4:3 aspect ratio section) is used to capture the image which is then downsized to 640 x 480 pixels. The effect of the crop video mode is to provide an approximate 7x focal length multiplier with little or no loss of image quality.
The maximum video file size is 4GB. This is a limitation of the FAT32 file system which is used to store data on the SDHC memory card. This is the same file system used on most Windows PCs and 4GB is the maximum size allowed for any single data file in the FAT32 system. With the Digital Rebel T2i in HD video mode, 4GB is enough to record for about 12 minutes. You can record any number of 4GB video files on a card, so you can record about 48 minutes of HD video on a 16GB CF card, but you have to do it in 12 minutes segments. In SD (640x480) mode you can record about 24 minutes in any one file.
To record 1080p HD video you need a card which can write at a speed of at least 330MB/s which means at least a class 6 SDHC card is required. Some class 4 cards might just be fast enough, but I'd advise getting a class 6 card since the price difference is very small.
Exposure in video mode is either automatic or manual (the T1i was auto only). In Auto mode the camera chooses the shutter speed, aperture and ISO and using center weighted metering. Exposure can be locked by pressing the "*" button (AE Lock). Shutter speeds between 1/25s and 1/125s are used. There is no continuous autofocus, but focus can be adjusted manually. Image stabilization operates as normal if an IS lens is used, but it's possible that the sound of the stabilization gyros may be picked up by the microphone in the camera if the external sound level is low. The audio is under AGC (automatic gain control), so that when things are quiet, the mic gain is increased which can lead to camera operation sounds being recorded. Contrast based autofocus during shooting is possible while shooting video, but it may be slow and there may be some overshoot or hunting.
In manual exposure video mode you can set the shutter speed, aperture and ISO manually. You can also set shutter speed and aperture, then with ISO in auto mode the camera will select the ISO setting which gives the correct exposure.
Videos are recorded as .MOV files (with MPEG-4 compression), and the sound is recorded using uncompressed linear PCM (pulse code modulation). The Digital Rebel T1i has an HDMI socket which can be used to directly connect the camera to a compatible HDTV to play back full HD video directly from the camera.
One word of note here is that if you play back a full size 1920x1080 HD video .mov file on a Windows PC, it will default to playing via the Quicktime movie player. On many (if not most) systems, the video will not play smoothly. This is not the fault of the camera and doesn't indicate a fault with the file. The PC (or more likely the video card) is simply choking on the amount of data which is trying to be played! It's also said that Quicktime isn't well optimized on the PC and works better on a Mac (which makes sense since Quicktime is an Apple product). However There is a way that I found to get smooth playback on a PC using the free open source VLC media player program which is available from http://www.videolan.org/. Note that once you have installed it on your system you may have you may have to do the following to set it up for smooth HD video playback.
The Digital Rebel T2i has the ability to grab still frames at any time while you are shooting in movie mode, but the movie is interrupted while the camera records the full resolution still images, so this isn't something you'd want to do if you were trying to shoot continuous smooth video.