Today is January 21st, 2009. I saw their other one at justin.tv/jonasbrothers, and I want to know when the next one is. Thanks!
Their video chat that happened a couple days ago
was the second one they've had in about 2 years.
So don't get too excited for their next one, although
they did say one of their new Years Resolutions was
to connect more with their fans so who knows they
could perhaps have another one this year but not
anytime soon.
EDIT: if it makes you feel any better I missed the chat
too 
I came home from shopping and rushed to the
computer and I was a couple hours late and it was
over. (Started at 5PM I got on at 7PM)
A long time away dude.
Sorry nobody knows the exact date yet. I got in late for the last one. And I missed the first one. But you'll find out a few days before it happens. Maybe in a year.
hopefully never
yuck