The Wikia says: [,,,] the interrogation with Stan has revealed that he has absolutely no link with the Death Eaters, but the Ministry kept him in order to put up a façade of progress.
ETA: Verification of the wikia quote:
‘They’re not still holding Stan Shunpike, are they?’ asked Harry.
‘I’m afraid so,’ said Mr Weasley. ‘I know Dumbledore’s tried appealing directly to Scrimgeour about Stan ... I mean, anybody who has actually interviewed him agrees that he’s about as much a Death Eater as this satsuma ... but the top levels want to look as though they’re making some progress, and “three arrests” sounds better than “three mistaken arrests and releases” ... but again, this is all top secret ...’
Half-Blood Prince - page 310 - UK - chapter 16, A Very Frosty Christmas
This bit from Goblet of Fire demonstrates that Stan is at least susceptible to magic that takes away one's free will or consciousness.
A third young wizard, whose pimples were visible even by the dim, silvery light of the Veela, now cut in, ‘I’m about to become the youngest ever Minister for Magic, I am.’
Harry snorted with laughter. He recognised the pimply wizard; his name was Stan Shunpike, and he was in fact a conductor on the triple-decker Knight Bus.
Goblet of Fire - page 113 - British Hardcover - chapter 9, The Dark Mark
And the Veela incident comes up again when Harry, Ron, and Hermione are discussing Stan's arrest and the likelihood of Stan being a Death Eater.
‘“Stanley Shunpike, conductor on the popular wizarding conveyance the Knight Bus, has been arrested on suspicion of Death Eater activity. Mr Shunpike, 21, was taken into custody late last night after a raid on his Clapham home ...”’
‘Stan Shunpike, a Death Eater?’ said Harry, remembering the spotty youth he had first met three years before. ‘No way!’
‘He might have been put under the Imperius Curse,’ said Ron reasonably. ‘You never can tell.’
‘It doesn’t look like it,’ said Hermione, who was still reading.
‘It says here he was arrested after he was overheard talking about the Death Eaters’ secret plans in a pub.’ She looked up with a troubled expression on her face. ‘If he was under the
Imperius Curse, he’d hardly stand around gossiping about their plans, would he?’
‘It sounds like he was trying to make out he knew more than he did,’ said Ron. ‘Isn’t he the one who claimed he was going to become Minister for Magic when he was trying to chat up those Veela?’
‘Yeah, that’s him,’ said Harry. ‘I dunno what they’re playing at, taking Stan seriously.’
‘They probably want to look as though they’re doing something,’ said Hermione, frowning.
Half-Blood Prince - page 208-209 - British Hardcover - Chapter 11, Hermione's Helping Hand
Frankly, this gives argument for either way: Stan could have been a Death Eater or he could have been trying to look important to his friends. I tend to err on the side that he wasn't a Death Eater. He didn't particularly stand out as gifted with magic. And in Deathly Hallows, during the Seven Potters fight with the Death Eaters, Stan is described by Harry as having a blank, unknowing look on his face.
He shot another blocking jinx at them: the closest Death Eater swerved to avoid it and his hood slipped, and by the red light of his next Stunning Spell, Harry saw the strangely blank face of Stanley Shunpike – Stan –
Deathly Hallows - page 55 - British Hardcover - Chapter 4, The Seven Potters
As far as I know, J.K. Rowling hasn't released any info about Stan; however, her website is currently down, so I can't check the FAQ.