On some further reading it becomes apparant that in Flash 4 and 5, this is legal:
Code:
var count = 20;
eval("identifier" + count) = "Dave";
// sets identifier20 to "Dave"
trace(identifier20);
But due to the increase in keeping with ECMA-262 specificaction you can't use eval() on the left side of an operator, so in MX, you have to use
Code:
var count = 20;
this["identifier" + count] = "Dave";
trace(identifier20);
If you try out the first code in MX you'll see that it fails.