You are using IE (IE5+ and above), so include include script that is specific for IE here.
You are NOT using IE, so feel free to use HTML 5 features.
So don't get lazy and assume that IE is history. While it's true that many savvy technical users have ditched IE for Chrome or Firefox, there are always users who will use IE because it is installed by default on their Windows OS.
Writing this entry made me curious about the current browser usage statistics. Amazingly, Firefox is way out in front with 42.9% this month, followed by Chrome at 25.6%. Microsoft's IE is still hanging in there with a respectable 24.3%. Still a contender but far from being the market leader in 2011.