Two different things:
1. In May there was a 2-week test of late night EMH. One park was open 1-3 hours late per night rather than the long-standing extra hour in the morning. Actually, they had a schedule were parks traded-off between opening early and closing late.
The May event was purely a test and there has been no word as to whether it will return or not. I don't really care to drag myself out of bed at 6:30am while I'm on vacation, so the late night hours would be much more attractive than the current EMH.
This was also a free event, open anyone staying at a WDW resort.
My wife and I were actually at WDW during the test phase. We did one late night at MGM. The results at that park were fairly predictable--RnR and ToT were immediately swamped as soon as Fantasmic let out. FastPasses were quickly gobbled up and standby times were near an hour.
I'd love to see some sort of late night alternative during the slower seasons. I'm just not sure how well this structure fits into the overall plan due to the variations in park closing times throughout the year.
2. During the summer months, Disney frequently holds E-Ride (or E-Ticket) nights at the Magic Kingdom. I believe they were held 3 or 4 nights per week through most of the summer. An E-Ride night is a separate ticketed event (about $12 per person). Admission levels are limited to just a select number of guests, and the theory is that those paying for this priviledge would have access to all of the highlight attractions with little-to-no wait time.
The last E-Ride nights were in August. There's no reason to believe that E-Ride nights will not return in June '05.
The fall months are not necessarily a great time for either of these events since the Magic Kingdom already has late nights for the Halloween party in October and Christmas party in Nov / Dec.