Update:(July 26 2007)To avoid this behavior you need to load your Flash via Javascript. For this you can use the SWFObject, I made an extension for Dreamweaver to ease the use of it.
In IE7 there is one more thing that they add that I find completely useless and unusable.
There is now a little info box telling you:
Click to activate and use this control
When you rollOver() a flash animation. It do load and play your animation but every first time you want to click on your Flash (on a button for exemple), the first click is to active the control and then you got to make a second click to interact with the Flash…
So I went to the Microsoft site where they use a very nice (lol) flash animation.
You can check their ie7Flash.js file that they use.
And so everything work without the info box !