The side effect was (and is) dyskinesias.

This breakthrough is related to this article also:
http://www.telegraph.co.uk/health/he...liminated.html

"Dr Politis said earlier trials showed the surgery could produce "remarkable improvement" in some patients but the majority suffered serious disabling adverse effects.

He said dyskinesia is a common side effect of drugs used to treat the symptoms of Parkinson's disease but the patients were suffering involuntary uncontrolled movements while off their drugs.
The new research, which is published online today in Science Translational Medicine, scanned the brains of two Parkinson's sufferers who suffered dyskinesia following surgery.

Dr Politis said researchers found the side effect was the result of an overabundance of serotonin cells in the transplanted tissue."
June, 2010

The article states that the increased seritonin is no big deal:

Researchers funded by the Medical Research Council (MRC) and Imperial College London now believe dyskinesia was caused by malfunctioning serotonin cells and say the disorder can be treated with drugs

So what drug(s) is he referring to???