Although it would be funnier if it just reset to zero.
ContextWeb does just that - anything less that $50/month or less than 50,000 impressions (are forfeited. I'll be anyone that signed up at ContextWeb and didn't read the fine print doesn't laugh at all.
Back to orignal poster:
Let's see, $5/month will pay out in about 20 months, assuming both your web site and AdSense still exist :)