Mine's not doing that. For example, I'm in Safari at the moment. If I click on "File" and then let go immediately, the File menu drops down and disappears immediately.
I'm now convinced it didn't use to do that. I remember that there was a slight pause between highlighting a sub-menu option (with a right arrow next to it) and when it would open up the new sub-menu. Invariably I used to click to open the sub-menu (I'm used to this from Windows), and the menu would just close, which frustrated me.
Is this an option I can change? I haven't found it yet. I wouldn't be surprised, this new Mac also had Dock magnification turned off by default, which I don't remember happening when I installed Jag from CD onto my older machines.
Home: 20" iMac G5
1.42Ghz Mac Mini
Work: 2x2Ghz G5 + 20" Cinema + 17"