I just graduated in June, but I still wear some of the more appropriate t-shirts to work out (fundraising events, some rush t-shirts, etc). I look younger than I am, so I don't think it strikes anyone as weird. I definitely look like I could still be in college.
When I move away and start a career I may retire the old shirts though, or just wear them around the house.
As of now, I've tossed the "Playboy Mansion" mixer t-shirts, though.
And the one with the naked leprochaun (sp) I only wear to sleep in.