Adding my 2¢, even if this is an old thread... I've thought a bit about this as many of my friends right now are casual pot smokers.
To me it would depend on the circumstances. If I tried pot once (I never have), I wouldn't change my sobriety date. Even if I smoked it a few times with friends, I would consider myself sober. (After all, alcohol was always my problem, nothing else). However, if I found it developing into a habit, I started smoking alone/etc... Then I would stop and change my sobriety date. IMHO, sobriety is about behavior more than anything else.