I bought a 28-135 right out of the gate and now I it's for sale. With the DSLR "crop factor" I am always frustrated with the wide end. The long end is good, especially with IS but there again I find that when stretching that far I wish I had just a little more reach. 28-135 seems to be an awkward range.
Lately, if I only want to carry one lens, I find myself choosing the kit lens 18-55 foregoing (word?) debateable image quality for better focal range.
I am activly searching for reviews on the Tamron 18-200 that has just been released as well as the Sigma 18-200 due out soon.
From what I have read a good bang for the buck is, and what I will probably wind up with if the Tamron 18-200 turns out to be a flop is
Sigma 12-24 ~500
Sigma 18-125 ~250
Sigma 70-200 f/2.8 ~700
Granted these are not "L" quality but from all the user accounts I have read this is a good "budget" mix. Especially for a noob like myself.
For your trip to Italy I would reccommend the Sigma 18-125 over the Canon 28-135.
The Canon is huge and heavy hanging from your neck (I have been wearing one for 3 months) Also for ~250 bucks you haven't lost a lot if it gets damaged. Also I would highly recommend a shorter length like the Sigma 12-24 to capture landscapes and architecture.
Here is a good overview in last month's issue of shutterbug of the 12-24
There is a really good post over on http://www.fredmiranda.com/forum/topic2/143791
featuring the 20D and the 18-125.
Hope this helps