irphotorumor, is it possible your issue is with AF accuracy and your camera is focusing past infinity? That seems more likely than the lens being soft at infinity. I've found Canon's cheap lenses don't focus very accurately on all cameras. Have you tried live view?
Fwiw, I've had great luck with the new kit lens. Corners are quite soft, distortion is present, and chromatic aberration is pretty glaring at close magnifications (but it's there to some extent on L lenses, too; I don't know why but it's everywhere on digital), but this thing is plenty sharp over most of its field of view. Its mtf curves are competitive with much higher end offerings and while it's 1/2 stop slower wide open than its expensive brethren, it's got three-stop IS. That is, of course, toward the wide end...it's not very good zoomed in, imo. But other than the semi-egregious CA, this lens is quite good, especially for landscapes, for which it provides a near-ideal range of focal lengths. If it weren't for the CA I would have no desire to upgrade, but I would love the 17-55mm IS if I could afford it.
I'm not sure I'd spring for the wide L lenses unless I had a 5DII, as they provide excess coverage (and thus physical size) for APS-C and lack IS. I might feel otherwise if I hadn't just blown too much money on a whole other breed of red-ringed lenses... And the 24mm TS-E looks tempting for any format.
I am sorry for effectively ignoring your post. I've been collecting some creative juices to reply you properly.
My EF 24-85mm @24mm loves my xSi AF module more than the given-away xTi's. The xTi (has, if I remember correctly, an older AF module) loved to (edited) back-focus @24mm, the behavior I did not love for my landscape shots on this end of the infinity. Still, on my xSi noticeable contrast is welcome for a greater number of focused shots. But truly, for 24mm shots, I set focus manually to about 5m and f/8 & 1/2 and continue breathing.
To get back on topic, I have not tested the AF focus at infinity using live view. I am kind a guy who shots RAW almost exclusively just not to worry about a singe setting when working the landscapes. I even started using exposure bracketing with continuos shooting just to avoid even looking at the LCD to check the quality of the exposure. This keeps my book-keeping a nightmare when I get back from shooting. What really helps is that I immediately go looking for the best shots, and there are only one or two of those on a good foggy day. They get my workflow attention the most. The rest, if I'm bored, I massage futilely later.
These real-life imperfections of the 18-55mm don't bother me. Perhaps a pinhole contraption on the xSi would be an interesting experiment.
I see there have been replies to your post already. I can't really add anything substantial (including image samples) to the discussion at this point.
"I'm not sure I'd spring for the wide L lenses unless I had a 5DII, as they provide excess coverage (and thus physical size) for APS-C and lack IS. "
In my amateurish experience, the technical quality (both construction and image) of the EF-S 10-22mm impresses me so much that the full-frame just seems unnecessary expensive at this point, even though i could drop down the dough for a 5DII and I am a proud owner of an EF 20mm already. The bigger viewfinder would be nice but I still shoot film with an Elan 7N so the-bigger-viewfinder ego trip gets experienced when too annoying to be put off. Also, viewfinder discussions make me laugh considering how many camera I have molested thus far. Those wanting a bigger viewfinder should get a 6x6 TLR :-)!
Best wishes and lots of fun shooting (and post-processing),