Has the 18-135 IS even hit shelves yet? or the 15-85IS USM?
Of the two the 15-85IS USM is the truly interesting one.
The 18-135IS does not have USM and hence I'd not consider it. Honestly, if it came down to the 18-135IS and the 28-135IS USM, I'd probably get the 28-135IS USM - simply because of the USM. Mind, you, I own 2 of those lenses (28-135IS) now and won't be using either.
The 15-85IS USM is an interesting lens to me, even with the 17-55 f/2.8 IS USM in my bag. I would consider picking one up if I were traveling to a less-than-safe place and didn't want to take my high-ticket items with me. The 15-85IS would take the place of my 11-16 f/2.8 and 17-55 f/2.8 IS USM in such a scenario - yes, I'd lose some capabilities (11-15mm, f/2.8-f/3.5-5.6), but gain some peace of mind. Its not just a cheaper lens than the pair, but its also less conspicuous. Its fairly compact, yet more capable than the 17-85IS or the 18-55IS's.