Forum Moderators: open
White Hats use commonly accepted techniques to promote sites to the search engines. These techniques are well within the bounds of acceptability and involve no subterfuge or trickery.
white hat:
calling your competitors by telephone and negotiating reciprocal link exchanges
grey hat:
finding old, forgotten Perl guestbooks with PR5 that don't escape HTML in comments, and putting things like iframes, meta refreshes and tightly packed linkspam on other people's pages
Tactics aimed at attacking other sites directly (rather than optimizing yours) such as page-jacking, DDOS, etc. are most definitely unacceptable under any circumstances.
If you are promoting your site rather than attacking others, then you just need to decide your level of risk. The major search engines publish sets of vague guidelines, which if you choose to ignore, may increase the chance of your sites being excluded from their index.