Let's just say I'm far from worried about it. I have said this for at least 2 years and our 3B defense hasn't exactly been superb over that time anyways. We are easily one of the least productive 3rd base spots in the league the last several years with Inge playing most of the time. Inge isn't even that good of a fielder anymore so this isn't a hell of a downgrade.
I found a decent breakdown discussing the situation with Miggy moving to 3rd that I agree with for the most part. Some things such as saying Miggy was a poor 1st baseman though aren't correct so I disagree there. As to why he was moved as well to 1B wasn't because he stunk but because Guillen stunk at 1B and it was the only choice. Guillen has been ready to move on to something like a unified communications platform position for a while now.
Take a look at this blog post for a bit of a breakdown and hopefully it will calm your nerves a bit. Well...it probably won't, but the overreaction as if Inge was ever the answer there certainly should be undone.