I have done this to a degree, although calling myself a commercial enterprise is quite a stretch, since it's just me. I have a subreddit which basically serves as a forum for my site. I chose reddit over a dedicated forum because I believe reddit has a better system than a traditional bulletin board.

I can't see an issue with it, even in the case of a large company, as long as they follow the rules like everyone else, no sockpuppets, no vote gaming etc. I don't think any company could gain enough subscribers to compete for an /r/all spot, even on a good day. (edit - meaning an officially-run subreddit, rather than a community-run one like /r/starcraft or /r/minecraft)

Several large subreddits already exist without direct involvement, /r/apple and /r/xkcd spring to mind. I'm not quite sure where to place them, they're somewhere inbetween regular communities and officially-run communities.

I imagine that most large companies prefer not to have such a public forum, directly on a social networking site, as they'd see it as a risk.

This comment is a bit all over the place. I should get some breakfast.