I guess ad placement would have to be a little different since it's a mobile layout. One somewhere above the fold, one later on, and maybe a guest rectangle between the first and second post on a page if you're not logged in, like it is on the main site. I'd rather not do the shitty ones that stick to the screen as you scroll, those are annoying.
Yes, floating ads do take up too much real-estate and are kinda buggy anyway.
How about the following:
We have a main ad right at the top like you suggest (just below the header) on every one of high traffic pages (main menu, view forum, view thread, view single post, view pm, view subscribed threads, settings, view user, etc.) The 320x50 example you provided would absolutely be the best format to use. It's attractive and doesn't eat up too much vertical space. When it comes to mobile sites, vertical space is at the most premium because of scrolling.
The only difference I recommend would be keeping this main ad off of the new post and new thread composition pages since a tap on an ad there could loose someone a lengthy composition. Mobile browsers are notorious for not caching what you've typed into forms.
Then, like you said, for users who are not logged in we display either a 200x200 or 300x250 ad after the first post in a thread just like you do on the main site. Logged in users would not see this ad.
As for having a third ad at the bottom above the footer, I'm kinda thinking that would be overkill. It's rare for mobile sites to have more than one ad per page request anyway and on some short screens (like the main menu) both ads could potentially be on the screen at the same time.
What are your thoughts? Once we agree on a plan, I can start working on some fake mockups to test out what the experience would be like.