if all your shots are going to me in daylight, you can get away without it. but if you find you're shooting below let's say 1/320 at 200mm you're going to be on that fine line of hand holdability. But of course, if you bump up to say 400ISO, you might be able to get away with it, but on the T1i, you might be starting to defeat the purpose of the high quality lens if you start to add noise by going too high on the ISO
The IS is a nice feature to have especially when heading into later in the afternoon light and you still want a large DoF, then it's almost necessary.
if you're using the hand canon for portrait work, and you always have flashes around, you can do without the IS
the golden rule for shutter speed and focal length is 1 over the focal length, so if you're shooting 200mm you'll want at the very least 1/200th of a second. even then, you're going to be on a very fine line for blur or no blur. When i have my 400mm on, and i'm not set up in a good standing position, i can barely put the single point autofocus on something without needing the IS to help me out.