DC1 changed nursery when he was 18 months old. I was so worried about taking him out of familiar surroundings. I took him out of the old nursery as I wasn't happy with the way he was n't cared for and the staff's attitudes. It was the best thing I ever did. He was much happier and so was I.
First nursery was from 9months to 3 years Second was for 2 terms (badly let down by prep school.......horrible situation thought we would be there forever....well until Year 6 anyway!) Now starting at the third nursery......but it wont feed into a primary school so he will move ready for school Sep 2012.
DS is nearly 4 and on his 3rd nursery. First one from 6 months to a year, second up to 2 and a half and now the 3rd until he goes to school. Each change was because we moved (from the US to London, then out to the suburbs).
He's not the most confident child and has recently had issues with being left out of activities at nursery, but I don't think it's anything to do with moving settings (but who knows really). Nothing we could have done about the moves anyway.
I would think it depends on your child and their age.
My daughter started nursery at 2.5 1.5 years later, we moved to Germany for 6 months during which she attended two different nurseries in two different cities. We then moved back to the UK (so a lot of moving around within 6 months !) and she attended pre-school for a couple of weeks before starting school that year. We have not moved cities since (only to a different house on the same road LOL) but up until her 4th birthday, we had been on the move at LEAST every other year.
She is very outgoing/adaptable and enjoyed making lots of new friends.
My DS (22months) started nursery a year ago and I wouldn't dream of moving him, even though he was a offered a place at a nursery much more convenient for me (was put on waiting-list, but opted out when offered a place as he had settled well into the first nursery). I'm not even sure I want him to attend the primary school's pre-school !
FWIW I think up until they start school, it's not such a big issue as long as your child has the personality to deal with frequent moves. My daughter definitely didn't suffer.