Here is the list of participants signed up according to fide.com (as of 18th May):
1. GM Rustam Kasimdzhanov of Uzbekistan (World Champion) 2. GM Michael Adams of England (runner-up of the Tripoli World Championship) 3. GM Judit Polgar of Hungary 4. GM Peter Leko of Hungary 5. GM Peter Svidler of Russia 6. GM Vishy Anand of India 7. GM Veselin Topalov of Bulgaria 8. GM Alexander Morozevich of Russia
Since this article is more recent, it could be more accurate but I could not find any other news source that confirmed that Kasparov was coming out of retirement.
