Lenses, unlike camera bodies, are made to last. They have much longer production cycles and also retain their value better if you want to sell them.
6 years is not old for a lens. Many of Canon's lenses are up to 20 years old and still popular today (eg 50mm 1.4, 85mm 1.8, 35mm f2, 70-200 f/2.
. Of course the cheaper EF-S lenses do get updated more frequently (probably partly because of the consumer mindset that you have to have the latest and greatest!) but the 17-55 is almost on par with Canon's L lenses and as such I think it will be around for a lot longer.
Even if it does get replaced tomorrow, this lens will still be a fantastic performer so there should be nothing stopping you from buying this lens now if it's what you're looking for.
edit: Does anybody know why f/2.8 is appearing with a smiley in it? I tried to remove it but it's still there...