[texhax] xymatrix question

[Michael Barr]

First, let me suggest that such questions are more appropriate for the 
xy-pic help group (xy-pic at tug.org) than texhax.  The answer to your 
specific question is to enclose the argument in a \vcenter:
\[\left.
\vcenter{\xymatrix{A\ar[r]&B\\C\ar[r]&D}}
\right\}
\]

This is also useful if you want to put an \eqno that is vertically 
centered, e.g.:
$$\left.
\vcenter{\xymatrix{A\ar[r]&B\\C\ar[r]&D}}
\right\}
\eqno(*)$$