My Bosch by-pass valve does both, it opens under vacuum for part or closed throttle and blow's open at about 6psi of boost. Its a fairly standard valve used on many turbo & supercharger applications, mine is the older style one; all thats changed over the years is they have stronger diaphragms in them now.
I'm probably just going to add another one. I was thinking of adding an electronic valve to stay shut upto 20% throttle which would open at 1volt (20%) from the throttle angle sensor, but its too complicated to make and too expensive to buy. I saw recently at a car tuning show an M45 blower on a IS200 Lexus that had just a manual by-pass valve connected to the throttle linkage.
I've also come up with an idea to make a baffled intake pipe similar to a straight through exhaust muffler box. I'll attach this before the K&N air filter and post my results if it works to reduce noise.