Not only do I think it could happen, I think it WILL happen IF...
1) Hundley struggles again next week vs the Saints. The Packers have a Week 7 bye, so the timing would be perfect to bring Romo in after Week 6.
2) Rodgers is out for the season. Romo won't want to simply keep the seat warm only to be replaced late in the regular season.
3) The Packers call. I think the Packers are more likely to say no than Romo is. He's a Wisconsin guy and grew up cheering for Favre. This would be a dream come true for him, and that's why I hope it happens.