Good observation! You just learned a fundamental property of lenses. In fact, almost *all* lenses achieve maximum sharpness at around f/5.8 - f/8.
Shooting at wide aperature will always involve some trade-off in sharpness. Wide aperature also have a very shallow depth of field, so your focus need to be extremely accurate.
I'm going to guess that it was a DoF issue, not a sharpness issue. If the AF was working properly, even F1.4 should lock right on to the correct focus area.
From what I've seen, DSLRs having the focus slightly off is not terribly uncommon, I've seen reports of examples of just about every DSLR having to sent back for focus adjustment. My first two seem to be spot-on but my newest is just a touch off and is currently away getting tweaked by the manufacturer (I had it fixed via the camera's AF adjustment but wanted it "perfect" while it was still under warranty.) For my initial tests, I used one of the standard focus charts you can find, but someone recently made a set of them that you can get
here. They're marked as being for the K20D but of course, they should work equally well with any DSLR. They even include a page that you cut and fold to give you the proper 45' angle, and include three different charts for wide, normal, and telephoto lenses.
However, if you're seeing issues with only one corner of the lens being off, than that may not be a focus issue at all. Only the actual focus point is "guaranteed" to be in focus, and with only F1.4, you'd need to make sure that the camera and whatever you're shooting are perfectly straight on. That particular lens has been noted a few times as being heavily biased towards center sharpness at the expense of corner sharpness - so you may just being seeing more or less normal behavior for the lens. But if it's under warranty and you can live without it for however long it takes to get checked out, it's probably worth sending it to them.