That's not a feature I've tested before. Using Page Setup does appear to center pages when I enlarge page sizes (e.g., from Letter to Tabloid).
However, using the Page tool with the Liquid Layout rule Re-Center selected should do the same thing. Just use the Pages panel to select all the pages you want to change. That could be more reliable because it was specifically designed to do that.
I've been doing this for years. Say for example I have a 20x30 inch document with a 20x30 inch box centered to the page, then I change my document size to 21x31 inches, the 20x30 inch box stays centered to the page.
For whatever reason, sometimes when I do this, the box will stick to a corner rather than the center. Can't imagine what's going on.
At least you now have a workaround, new in CS6.
A workaround is nice yes. I would like to figure out what the deal is though, if it's a bug or something I'm doing.