Rather than using a Canon/Nikon own brand lense there is an alternative, Ive owned a Sigma 18-200mm for my EOS400D for 2 years now and am very impressed with the results 18-200 is roughly 28-300mm in 35mm terms and this lense has an image stability function in the form of one of the internal elements activly moves to stabilise the image.
Im very impressed with this one, beats the pant off Canons own lense, highly recomended, has hardly ever been off the camera since i got it.
sigma 18-200mm lens - Jessops
Shop around for the best price and there is also a cheaper version on the market without the image stab bits.