From a quick survey around the place, it seems to me that the default phpBB theme (subSilver,) and the great majority of other themes for phpbb, all have the submit button on right. vBulletin's default theme also has the submit button on the right. However, Invision Power Board's default theme has submit on the left. I wonder why they decided to be so ugly and different. (Btw, Phpbb, vbulletin and IPB are the three most common forum software packages.) If you use a cool browser like Firefox you could probably do some custom stylesheet floating to switch them around.. although I wouldn't know quite how to do it.