Let's look at some facts from the previous years and see what is the pattern right there...
FM2010 Demo = 14 Oct, Game Release = 30 Oct
FM2009 Demo = 02 Nov, Game Release = 14 Nov
FM2008 Demo = 30 Sep, Game Release = 18 Oct
FM2007 Demo = 30 Sep, Game Release = 18 Oct
FM2006 Demo = 30 Sep, Game Release = 21 Oct
Every year (from FM2006 onwards, at least) the game is being released in October, with the exception of FM2009 that was delayed for a couple of weeks.
The time margin between the demo and the full release is indeed roughly 2 weeks, sometimes less and sometimes a bit more. I believe the day matters as well.
As I explained here, the game is always getting released on a Friday because it's the perfect timing for having a full weekend to play, either being kids returning from school, or adults ending their working week.
As expected, the official release date of 5 November is a Friday, and with the same logic the other candidate day for release (29 October) is also a Friday... coincidence? I don't think so. Doing the math and going 2 weeks backwards and finding the closest Friday there, I see this:
22 October as the most probably release date for the demo.
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UPDATE 18 Oct: 22 October is really the official release date for the demo. However, Steam users will be able to get strawberry version a day before. Read all the details here FM 2011 Demo release dates confirmed.
UPDATE 16 Oct: It's almost certain and confirmed that the demo will be released next week. And while 22 Oct remains a good candidate day, there is a chance we may see it a couple of days earlier so that could be 20 Oct. At any case, I wish to thank all readers for making this page so popular (over 10k views and counting), and I hope it proved a good companion all those days while waiting for the demo.
UPDATE 13 Oct: Composed a blog entry full of nonsense comparing the 2 flavors the demo usually becomes available to download... turns out Vanilla gets more points (?).
UPDATE 11 Oct: The official response from SI on what came up with play.com (and the rest) is that there has been no change of plans, so 5 Nov remains the scheduled release day. Miles also said that the demo will hopefully be out by the next podcast... which is on the 22nd!! It seems my prediction is spot on so far!
UPDATE 10 Oct: Gamestation has also confirmed that the release date has been moved a week sooner to 29 Oct, so 15 Oct seems quite possible for the demo now.
UPDATE 9 Oct: According to play.com, the release date for the full game has been put back to 29 Oct (?), so it's possible that we may see the demo coming out on the 15 Oct (!).