Yes, it's quite common for the picture to have substantially less depth of field than you see in the viewfinder. I'm not sure specifically about the T3i, but for at least some cameras you can get a different viewfinder screen that diffuses the light more, so it shows depth of field more accurately. For what it's worth, that also tends to make manual focusing easier (things that are out of focus look out of focus). The downside is that the view through the viewfinder gets somewhat dimmer.
Here's a demonstration of what I'm talking about. These are taken with a Sony Alpha A900, but the principle is the same. Both are taken with an f/2.8 lens at f/2.8. In both cases, I focused so that in the viewfinder, the
135 was somewhat blurry, but still readable. Here's what I got from doing that with the stock ("type G") screen in the viewfinder:
Here's what I got when I used a viewfinder screen with more diffusion (what Sony calls "type L"):
I'll repeat: In both cases, I used manual focusing, adjusted so that the
135 (just before the big
36) was (somewhat) blurred but still readable when viewed in the viewfinder. In both cases, I was using an f/2.8 lens at f/2.8. In both cases, we're dealing with what I saw through the viewfinder, with an f/2.8 lens at f/2.8, not a faster lens, nor using DoF preview, nor comparing the view through the viewfinder to a (larger) print.
So, one more time, all together on the count of three now: yes, the viewfinder screen does affect the apparent depth of field, even with an f/2.8 lens, even when it's wide open.