A post on Bitcoin Miner blog describes a chart from BitcoinX showing the revenue that is produced each day per 100 MHash/s of mining capacity. Excerpts:
"Those hoping to pay off their rigs after a few months are going to be frustrated".
"Those who are more efficient or pay a low amount for electricity are still adding enough capacity to offset all those calling it quits".
"AMD has beefed up the production line for [the Radeon 6990] graphics cards and stocks of these cards will likely be getting replenished soon".