Bonus AS stacks additively, but the formula to calculate it is a little weird.
http://leagueoflegends.wikia.com/wiki/Attack_speed
However, if you work out the formula for someone, say KogMaw, you see that AS progression is linear.
https://docs.google.com/spreadsheet/ccc?key=0AmzyRzukH7kbdG9ZbkRxeXFPSXdZR2c1ZmIxMWRmbFE
So, every point of AS% is worth as much as the last, up to the cap (2.5 attacks per second).
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"What if you just eat vegetarians?"-neonreaper
"^ You are what you eat. ^_~"-Koiji