This is a brilliant, simple solution - there are several "AI" cart abandon programs such as Upsellit.com, and other live-chat solutions, but Olark's inclusion of user data, cart value, and simple IM integration will make a big difference.
I'm 100% going to test this, and will report back about conversion results vs. other live chat or AI tools.
This is another one of those things that you'd figure people would hate and be annoyed by, but actually ends up being really effective. A company like ThinkGeek probably couldn't leverage it, but someone running a CPA weight loss offer definitely could. In a large way.