I don't think Captcha is ever locally generated, otherwise it would be very easy to "crack" the captcha as someone could just run a script to figure out what the correct text should be.
I am interested to see if this explanation will be accepted by the defense and the court.
There are plenty of implementations for locally generating captchas (cool-php-captcha is a popular one). Bitcointalk uses the stock SMF one. They are obviously easier to crack than something like recaptcha, but they do provide some kind of protection. Most Tor hidden services use them, it's generally a bad idea to include external elements on a hidden service and most captcha services use javascript too which is problematic with TBB.
It's a very plausible explanation, in fact according to the FBI Ross had written in his logs that on a number of occasions he had found/introduced similar IP leaks on SR and the SR forums and had to move servers a couple of times and the alleged dates of these incidents seem to coincide with downtime of SR and SR forums.
On top of that it looks like Christopher Tarbell was on this case, he has made a name for himself having taken down many high profile cybercriminals - in fact even for similar mistakes such as this, Sabu (LulzSec hacker) was caught by Tarbell because he leaked his IP when connecting to an IRC chatroom,
I'm not sure why they kept the method of discovery of the server secret until now though. I'm still not entirely convinced Ross is DPR either, their evidence on this end is HIGHLY circumstantial IMO.
They appear to have caught him red handed with his laptop open and logged into SR at the San Francisco Library. Sure it would be possible that he was just a moderator that was hired after it opened, but this would not explain how his email address was used for the forum account that posted the first advertisement about silk road (I think it was a weed/drug related forum).
The question is will this be sufficient evidence to prove beyond a reasonable doubt that Ross is in fact DPR, my answer is, I am not sure. The question is was Ross in fact DPR, I would say yes, or at least he was somehow involved in SR at a very high level.
You should remember that the FBI apparently was able to seize several thousand bitcoin when they got his laptop (although this also may have been the result of seizing the servers in iceland).