We just developed an app called RingGate that enables you to select contacts that will audibly ring thru while your phone is on silent/vibrate. So (i.e.) you're in a meeting and have your phone on silent, but you're waiting for a call from the mrs (or a better job offer)......or, you want to get some sleep but still want your buddy/girlfriend/kids to be able to get ahold of you.
A new additional option is to allow your most recent call to automatically (if you choose) ring thru for a set period of time in order to avoid the dreaded phone tag phenomenon.
Check it out on android market. There's currently a free 2 week trial on the full version. Hope you like it.