Delta can ticket 331 days in advance. (if Skymiles seats are actually loaded into the system at the correct time and show availale) Many travelers who use their miles frequently are aware of this and will call that far in advance to hold or ticket the seats. Delta, believe it or not, is number one in the industry - giving more seats away through their Skymiles program than any other US carrier.
Also remember that many travelers will put a reservation on a wait list, so if someone else cancels their tickets - or their hold expires, these seats will go to those who were waitlisted.
Although Skychoice may seem like a rip-off to some, it does allow passengers the option of flying on a flight that has already sold out of its' Skymiles seat allotment. Think about it, if 10% of a 199 seater 757 has Skymiles seats and they're already gone, technically, the rest of the seats could be sold using Skychoice. That means Delta is not making the revenue that it would have had it actually sold seats on the plane. The benefit for Delta, is that you've given up those extra Skymiles, and thus will not spend them on another free trip.
If Delta notices that a flight looks very empty as it draws closer to its' departure date, it is very possible that more Skymiles seats will be released. I've always found Skymiles seats to Hawaii 331 days out, and within a week or two. The bad thing about Europe, is that Delta is really moving out internationally, especially out of Atlanta. That means MILLIONS of Skymiles members are earning miles flying their dreaded business trips from Cincinnati to Chicago every day, but they all want to go to the same places for vacation... Ireland, Hawaii, etc. (never judge availability be a seat map on a web site... as passengers, we 1)do not see the entire map and 2)don't know if those that are already ticketed even have seat assignments - common with travel agents ticketing reservations)
Check out the details on their website. Fees may apply if you ticket within a certain amount of time prior to your departure date, making it better for you to ticket in advance. Also, Delta has offered something new where you can ticket Skychoice there and SkySaver back, or visa-versa. You can even buy a one-way ticket, I believe.
If you purchase a ticket, as stated previously by another, it must be in a certain class of service to be eligible for an upgrade. I do know that you should be able to upgrade at the time of booking... so it is not as if you have to chance it once you check in the day of departure. The miles however, should be taken out of your Skymiles account by the airport personel the day of departure.
I've tried their Business Elite internationally and I'll have to say I LOVE IT. With my personal experiences, my trip from Atlanta to Amsterdam was the best on Delta.
My last word of advice on Delta, if you do call in to talk to a representative, make sure you feel confident with the one you are speaking with... hang up if you need to and call back. Delta does have great service, but like any company, bad apples can exist in any apple tree.
Hope this helps... Good Luck with your Trip!!!!!