When there is too much work for one scrum team to manage, multiple teams are created. A scrum team has a stand up meeting everyday. How often should the intra-scrum team meeting occur and who should attend? What should they talk about?
We have Scrum-of-scrums daily. The typical day goes like this...
9:15 - individual teams have their stand-up meetings. The usual stuff: what you did yesterday, what you're doing today and (most importantly) any blocks 9:30 - Scrum Masters from each team have their Scrum-of-scrums. Same stuff: what the team did yesterday, what they're working on today and most importantly (even more so than for the individual team Scrums) any blocks and/or cross-team issues
Doing it this way requires a lot of discipline to stay focussed on the important stuff, but that's Agile in general for you.
For me, the most important thing, regardless of how or when or who is that blockages and cross-team issues are raised and resolved quickly.
Finally as Casey mentioned, don't be dogmatic. If it doesn't work - do something about it.
Initially, I would plan to have a "Scrum of Scrums" once a week, preferably on a similar schedule to your planning sessions and retrospectives. As far as who should be in attendance, I would advise you to have each team elect a member that will participate and speak on behalf of the team, but leave the door open for any team member to attend should they want to.
The beauty of Scrum and Agile methods as a whole is that there is no single magic recipe. The advice that I gave about the schedule and attendees should be fluid and easily modified. The best kind of agile, at least in my experience, is organic agile. It's difficult to be truly agile when you're dogmatic about your processes. ;)