Tijn said:
The new 70-300L IS (2010) has one stop better IS than the DO, it's as sharp as the 70-200 f/4L, it has weathersealing and at the longer zoom range is a bit slower than the 70-200 f/4L. Length wise it fits somewhat in between the 70-200 f/4L and the DO. But weighing about 1kg I doubt you should consider this as a travel lens.
I would totally disagree - I find the 70-300L an excellent travel lens, the 1/2lb difference in weight is neither here nor there - but its compact size is relevant when putting it into a small bag. The extra reach over the 70-200 f/4 makes it far more versatile. If you have a crop body then this coupled with a 17-55 f/2.8 makes the ideal pairing for the minimal travel kit covering 17-300 with just two high quality lens.