Yes. The incidence of tents being damaged by foxes in search of food is increasing. Reports from the trekkers suggest that attacks are becoming incresingly co-ordinated. Strange this, as foxes are by nature solitary animals.
Keep food away from tents or deep inside a rucksack. Keep objects away from the inside of tent walls as this can encourage the fox to try to enter. You may also try a homemade pepper spray which has had some success. Apart from that, take a dog!
More info: check out the main forum for related "fox" posts.
Also have a look at the following
http://www.sierra-nevada-news.com/2009/07/coordinated-fox-raids-on-trekkers-in-the-sierra-nevada/