Completely true and believable. Derren Brown uses
Neuro-linguistic programming to affect those types of changes in people on the show, not mind control. It's really interesting, my brother and I have been studying it as a hobby for about a year now. It all has to do with the way your senses interpret the world, and how strongly communication with other people is tied into that perception, into thought, even consciousness itself.
One thing that even Derren Brown admits is that different people are affected differently by it. Some people are incredibly susceptible while it just doesn't seem to work at all on other people. Many of his best NLP scenes take dozens of takes to find someone who is fully susceptible.
Youtube has many more incredible examples of NLP at work, most are from Derren's shows, but there are others. Search for NLP or just Derren Brown and you'll find some awesome stuff.
EDIT: Went looking and found some of my favorites:
Affect a seemingly random choice:
YouTube - Derren Brown NLP
This poor guy thinks he's actually in an arcade game:
YouTube - derren brown zombie
Induce drunkenness without alcohol:
YouTube - Derren Brown Simulating Drunkeness
Get a winning payout at the dog track with a losing ticket:
YouTube - Derren Brown at the Dog Track
Here's one that explains the techniques Derren uses in the dogtrack video:
YouTube - Derren Brown at the Dog Track (explained)