No hope for a pre-release demo, I am afraid.
All will happen that Take2 will give the game to IGN, Gamespot and the like some 3-4 weeks before release to write the hands-on pieces, see:
IGN
Civilization IV Review - October 21, 2005, Review
6,000 years of fun packed into a single game.
Oct 7
Civilization IV - October 07, 2005, Preview
From BC to AD, we go hands on with the upcoming sequel.
Gamespot
Civilization IV Review
If you have even a passing interest in strategy games, world history, or getting less sleep at night, you owe it to yourself to give Civilization IV a try.
Review - Posted Oct 26, 2005 6:04 pm PT
Sep 20-22
Civilization IV Hands-On - The Middle and Late Eras of the Game
We play around with the middle to late parts of a Civ IV game to get a feel for what to expect.
Preview - Posted Sep 22, 2005 7:00 pm PT
Civilization IV World Exclusive Hands-On
It's one of the most anticipated games of the year, and we played the heck out of it.
Preview - Posted Sep 20, 2005 5:11 pm PT