Yes, I believe that a number of blenders have "defaulted." Prosper, of course, did nothing.
There were a number of Prosper statements and promises and drama after Andrew's March 2, 2007 forum post that you quoted. As best as I can recall:
A few months later, someone asked for an update. IIRC, that may have been Leporello, and that thread may have included a table of late blenders. Prosper ignored this for a while, and eventually there was a forum shitstorm.
Prosper (Shira, I believe) then said that Prosper had instituted a change -- late blenders (possibly they had to be 1-month late, I can't quite remember) would not be able to self-initiate outgoing transfers through the web site. However, if they called Prosper, Prosper would do so for them. In addition, late blenders could not bid on listings.
Later, someone (Leporello I think) posted an example of a late blender who had successfully bid on a listing. Prosper explained that for the time being their new restrictions on late blenders had to be imposed manually, which Prosper intended to do once a month. Thus, depending on when in the month (compared to when Prosper did the manual processing) a blender went late, it was possible for a blender to be fairly late before the restrictions kicked in. I think this was also the time that Prosper suspended Leporello from the forums for this, later claiming that the suspension was a mistake after a huge firestorm on the forums (including my own giant red signature all over their forums about Lep's suspension).
Then around late-summer 2007, Shira posted that Prosper's lawyers were studying how best to include a right of set-off in the legal agreements that would permit Prosper to seize late blenders' lending accounts and pay them to the lenders on their loans. She promised either to have this implemented, or at least to provide an update on the lawyers' work (I can't quite recall which), by "the end of the year." Xraider and I posted our beliefs that several months was a ridiculous amount of time to have to wait for this, especially since it had already been months since Andrew's March 2 promise that Prosper would pay lenders with late blenders' accounts (and I'm pretty sure that the late blender issue had actually been raised on the forums (perhaps by Urbi or Epic?) in the summer of 2006!). Shira never explained why it would take until the end of the year (2007), and of course, as per usual, nothing was ever heard from Prosper on this again.