I believe it's not when it starts, but when it ends that the end of month thing matters. You can use it as soon as you get it (or the temp card you can pick up at Epcot). However, as long as you order after a certain day of the month, it's valid longer--until the end of the next month, in the next calendar year. Clear as mud? LOL
In other words, don't order it until you absolutely need it if you think it's possible you may use it again in the next calendar year at the same approximate time. It will be valid longer if you wait longer to order it. It's not like the APs that are valid for exactly one year from when you first use them at a park. DDE is good for 12 months from when you order it--though at the END of the month--and if you order after the first half of the month (not sure the exact day, it may be the 10th) it won't expire until the END of the 13th month from then. (so, for example: if you order it today, it will expire Nov 30, 2008, but if you ordered it on October 1st, it would expire on October 31st, 2008)
Oh, and it's worth it if the total amount of $$ you spend on food and drinks is over $325--because 20% of that total is the cost of the DDE. It's more than worth it to us. Here's a thread on the "worth" question:
http://disboards.com/showthread.php?t=1609220
Don't forget to make sure the places you want to eat and drink take the DDE when you are figuring out if it will be worth it for you.
Oh, and you can technically get it at WDW--just pick up the phone (M-F during business hours) and then go pick up your temp card after 5pm the same day at Epcot guest services.
here's a link to the DIS page with info on DDE including lists of all the restaurants that are included:
http://www.wdwinfo.com/disneydiningexperience.htm