Canon 17-55mm f/2.8 – tilted lens element?

Thank you both for your advice.

Sorry to hear about your bad experiences, dave_bass5. I hadn't realised this was such a problem with this lens – to be honest, tilted elements were one of the reasons I'd leant towards the 17-55 over the 15-85.
I've tried to give some thought to not pixel-peeping but, now I know it's there, I'm unfortunately conscious of it even when viewing at full size. After much deliberation, I contacted the company I'd purchased it from who, as always, were fantastic and have offered a replacement. I'll keep my fingers crossed for the replacement being unaffected.

The way it shows more prominently on certain photos is quite strange – when I tried a simple brick wall test, everything looked OK. But it seems to show up most beyond a certain distance and not necessarily on the things closest to the lens. This is the far-left edge:

Is there any possibility this could be caused by field curvature? (Apologies if I'm way off – as you can probably tell, I'm learning as I go along... I'd just read in reviews that field curvature was particularly visible at 17mm and wondered if that could explain the results I've been getting – and why the smudging occasionally varies depending on an object's angle to the lens. )

Thanks for your suggestion, plantdoc – when I checked the rest of the focal range, it seemed to only affect 17mm. I must admit that I've only checked the lens wide open, but I'll have a look at any photos taken at f/4 or f/5.6 to see if it's showing in any of those.