I also know nothing of the deal, but think your frame of reference is off.

At this scale and early stage -- both companies appear to be pre-any-revenue -- both teams should be most interested in the promise of long-term equity appreciation. Posterous isn't rolling in cash, and rather than shuffling around any cash balances either company has, all cash should be going to a longer runway. A pure stock-and-job-offers deal seems most likely.