Most web sites that I see are centered, and I'm wondering if there's some sort of unwritten (or written) trend away or against left aligned sites?
I don't have current statistics, but studies done by Smashing Magazine in 2008 and 2009 found that 94% of popular blogs and 89% of design portfolios were center-aligned. It's a small sampling of websites, of course, but it indicates that the overwhelming trend is toward center-aligned websites.
If I had to venture a guess, I would say that the reason for this is because of increasingly bigger monitor sizes. Most web designs simply don't look good on 20-23-inch monitors unless they are centered.
Yes the trend is more for centre design. In the past left align was more common when the screen where smaller (and a bit more difficult to achieve, you were using tables to create centre alignment). Nowadays centred align make more sense because you can balance the empty space on left and right filling better the screen.
I think that left align can be still used for interesting play with background (like graphical mood effect, or advertise), but you have to be clever and inspired to achieve a good effect