Here's a good, reasonably comprehensive list of open source events in 2008. It's missing some (like OSCON) but is intended to get better through community input. So, if you don't see your favorite conference there (Ubuntu Live, anyone?), let the site author know.
By the way, if you're a commercial open-source company, you may have already discovered what Alfresco has discovered: the open source events aren't very useful as lead generation tools. People don't go to Linuxworld looking for a content management system, for example.
We've found a lot of success, however, at conferences aimed at our product category (like the AIIM conference). Why? Because an open-source company at a Business Intelligence conference will really stand out, for example, while that same company won't make much of a ripple at Linuxworld.
If your intent is to establish yourself among your open-source peers, however, it's a different story. Open source events make sense then. Also JavaOne has been very good for us, and it's a bit of an open source show now.