Maybe someone should run a query on the data dump to see whether uppercase or lowercase is more common for the first character of a comment. My suspicion is that lowercase is more common.
Update: As balphabalpha indicated in a comment, upper-case is far more common:
From the SO March data dump (with the crippled comments, but statistically it should be okay): 1,934,139 comments total, 1,425,013 (73.68%) start with an uppercase letter, 247,919 (12.82%) with a lowercase letter, and 261,207 (13.51%) don't start with a letter at all.