There are three trusts and a foundation return, as well as the personal returns, of the Romneys on their website. Well, all as filed for 2010 and pro forma individual only for 2011. I don't know how you'd compell a partnership or other entitiy that includes non-Romneys to release their tax returns.
The thing I find interesting is that the Romneys are stating their effective tax rate. My clients usually talk about their marginal tax rates. "I'm in the 25 percent rate, right?" "I'm no longer in the 25 percent rate after my last raise." So, they think 13% sounds pretty low. I have to remind them that their effective rates are 6.9% or 11% or even 0%.