According to what appears to be an official press release given to OnlineFlorida.co.uk, we can confirm 25 nights for this year's event. The dates are posted below. The release also notes the name of the event as Halloween Horror Nights XXI.
"Combo Ticket Admission (includes Day and Event admission):
2-Park Bonus with Halloween Horror Nights® Combo Ticket. The 2-Park
Bonus with Halloween Horror Nights® Combo Ticket entitles one guest
admission to Universal Studios Florida®, Universal’s Islands of
Adventure® and Universal CityWalk® on all days during a fourteen (14)
consecutive calendar day period which includes the first day any portion
of the ticket is used.
This ticket also includes admission to HHN during the following 2011
Event Dates which fall within the fourteen (14) consecutive day period:
September 23, 29, 30 October 2, 6, 7, 9, 13, 14, 16, 19, 20, 21, 23, 26,
27, 28, 30, or 31. Not valid for any Saturday HHN Events Dates:
September 24, October 1, 8, 15, 22, or 29. "
Also, it appears that Universal has returned to Wild Indigo for their TV spot. Hopefully this means the return of Dean Karr.