Two additional commenting thoughts - probably more applicable to "front-end" code, but also somewhat to TSQL:

1) If coding to a design spec, reference the section of the spec explicitly.  Instead of just "this gathers data for the month-end report", say "this gathers data for the month-end report (see section 5.3 of xxxx spec)"

2) If fixing a bug, reference the bug number if applicable (for example:  "PE 1/20/2009 - fix for bug #43 - C-in-C Error")