It happened to me once.
Using a good escrow agent, such as Moniker, can help to avoid this.
In my case, eNom simply reached into my account and snatched the name back. Of course, there was no proper process.
Memo to eNom: 40(# you!
The domain broker, a Canadian college student named Chris, quickly hid under a rock.
It's one reason why I think allowing domain "brokers" to post on boards is bad policy.
What is the sellers' responsibility in all of this? If they are leaving their passwords around or using obsolete email addresses or keeping their domains at registrars with poor security --do they bear any reponsibility?