Thanks for all the replies guys. Although I have limited knowledge on the two lenses, my own thoughts were that for landscapes the IS and f2.8 were not required. However some info I have seen on the 17-55IS appears to suggest that the IQ is fantastic, although from reading your replies it would appear that the IQ for both would be on an equal par.
As a matter of interest, the body I am looking at as a possible upgrade is the 50d (unless I win the lottery!). At the rate Canon are churning out new bodies, the temptation is to wait and see what comes next, or what existing ones fall in price, but I suppose that is a discussion for another thread.