From United's site:
FA Cup Final, Manchester United v Millwall, Saturday 22 May, kick off 3pm
The deadline for Private Box / Executive Suite and Club Class Members was 5pm on Friday 30 April and the initial registration period for qualifying Season Ticket Holders has now passed.
We anticipate that approximately 2,000 tickets will remain and Season Ticket Holders who have purchased a ticket according to our records for the following 6 cup games Panathinaikos (31) Rangers (32) Stuttgart (33) Man City (34) Fulham (36) and FC Porto (35) from this season (2003/2004) will be able to register an application by telephoning the Ticket Order Line on 0870 442 1999 from 8am on Monday 10th May through to 5pm on Tuesday 11th May.
A full announcement regarding prices etc will be made on Thursday 6 May.
Please note, we anticipate that a heavy volume of calls will be received in the first couple of hours and that supporters may have difficulties registering during this period, therefore please be aware that this is a 24 hour service and tickets are not allocated on a first come first served basis. We anticipate being heavily oversubscribed and an automated ballot will take place to determine successful applicants.
All payments must be made by either debit/credit card, however please note, if successful an administration fee of 3% per transaction is charged for this service.