The IRS goes fishing daily and never comes home without a catch. Anyone who has been under their spyglass will tell you it doesn't matter if they find anything or not (and they will), it's the anal probe you get in the meantime that smarts.
It would still be a fishing expedition. I have had a MtGox account for months but I never used their platform for a single trade.
I think the point should be made that Mt.Gox was not upfront in saying the username database had been stolen. The single account that was hacked or used is one thing but the release of 61k accounts with or without the password hash being crackable shows a real security concern with the site.
Second this is a goldmine for the IRS as the value of Bitcoins has increased and those who have been trading or hording from the first days and have made a profit must realize income is income to the IRS (US, you insert your own country) and sooner or later it will draw their attention. Now instead of them having to subpoena a server owner in some foreign country they can just subpoena Google for all those GMail accounts listed. Once they start that fishing they won't stop which makes me curious about the username order in the leaked file. If I was in the first 200 and one was to assume the order is oldest to newest and one must also assume the oldest have gained the most, well I might be double checking my tracks for emails related to profit, sales, etc.
For those with business email addresses listed I also would not assume your bosses won't get this list and if your using company resources for mining or anything else I would cover those tracts pretty fast too.