If I am traveling I want IS because I will not allocate enough time to make sure the picture comes out perfect and I am not likely to be at my steadiest since I am trying to pack as much fun into the trip as possible.
For this purpose, the Tamron 24-70 should do fine, too - at least it's $1000 cheaper and thus it's only half the catastrophe if it gets damaged.
I start wondering if it would not be a bad idea to let the first batch go when it becomes available....
For once, I have to speak up for Canon: obviously they delayed the lens the last time due to beta testers' feedback (whatever that was), and given the total delay anything other than a 100% perfect qc would be a total marketing disaster.
The one possibility is that the cps samples they're shipping now are hand-picked and there might be another slight delay for the normal customer - say until September 2013 or so :-p