Sgt_Kickaxe - 12:30 am on Feb 13, 2012 (gmt 0)
We made some changes to our blog url structure on Jan 20th 2012 and I am curious if this is where we ran into trouble.
This could absolutely be the cause if you mean you changed your internal link structure, changing the url structure in effect shifts the incoming link value of each page in ways you probably can't predict. While I'm sure you kept your most important pages less than two clicks from the index page, hopefully many of them a single click away, the impact of wholesale changes can't be known until the dust settles. It's also possible that just making changes has, at least temporarily, given Google reason to want to re-evaluate the site before trusting it on page one again.
Not all is lost. Because it takes time for all changes to propagate fully it's likely you'll see some pages slowly rise back to their prior positions all on their own after google is done re-evaluating the site and it's new internal link structure.
The only major change I would even consider is a full reversal back to the original. If that's not on the meny proceed slowly right now and make only minor changes with a good 10-14 days or more in between.
The url and title tag structure were changed.
If I understand that correctly you changed the actual url to be different from the old urls. By doing so you have essentially created all NEW pages that Google does not know. A 301 redirect now passes all incoming link value to the new page but you don't get a 100% value through a redirect so you will take a rankings hit if all your pages suddenly did this.
In either case I don't think this is Panda, it's a case of too many changes at once and an actual change of urls (always a last resort).