I REALLY don't think Fudge is a Death Eater. (IMHO, he's just aan idiot that didn't have the courage to face facts until they were shoved in his face.) However, I could see how someone Percy respected turning out to be a Death Eater could bring about that change.
I think any fall to the Death Eater side with Percy would have to be gradual, over the course of a whole book, even. I think he has personality characteristics and experiences that make willingly becoming a Death Eater very plausible given the right circumstances.
JKR could put him under Imperio but I think that would take all the fun out of life.
Oh, and I wonder if Professor Umbridge's allegiances were really to Fudge and the ministry. What if she was using Fudge's idiocy to serve Voldemort?