Not that these events seem to need any more promotions given the rate they are selling out, but I'd think a badge for attendees of a DevDays event would definitely reflect people that are active members of the SO community.
I'm going to go against the grain and say: I think it's a good idea. (And no, I can't attend any)
The argument against it seems to focus around "I shouldn't have to pay for a badge" and "I don't have the opportunity to go, it's not fair." Another one I suppose would be "Badges shouldn't be awarded for Real-Life Actions."
The arguments for? Badges are meant to convey your contribution and interest to SO. Contributing to a real-life (not-for-profit) extension of Stackoverflow, Dev Days goes along with that. (1)
The Beta Badge fits against "Badges I don't have the opportunity to earn". The Hacker Badge, and the few people who have earned it, fits against "Badges I don't have the opportunity to earn", and to a lesser extent "Badges shouldn't be awarded for Real-Life Actions."
You will not earn every badge, and it's not unfair to limit you by awarding badges to others who have contributed in a way you can't.
FWIW I still think a Founder/Developer badge is also fine.
(1) AFAIK Dev Days is not for profit, or at least not for much profit. StackExchange is radically different.
A badge for attending isn't a bad idea at all - similar to the Beta badge in that it's a one-time deal and can't be earned if you miss it. Maybe I'd feel differently if I were unable to attend, but I don't think that it's necessary for all the badges to be winnable and/or no barrier to entry (i.e. no-cost).
I envision the badge could look something like this:
with the format being
How many people are going to ask this?
The answer has always been 'No'.
I don't expect that to change.
Good for Building Community
It would be a nice way to put a face to a user.
"Hey, I remember meeting this person...."