By my calculations, USD 0.507 in electricity costs per btc mined.

The assumptions:
Power efficiency: 8.3 J/GH (ref: https://bitcointalk.org/index.php?topic=204030.0)
Electricity cost: USD 0.06 per kWh (was mentioned by someone earlier in this thread, cannot find official confirmation)
Current hashes to win: 91635939047418116 (ref: http://blockexplorer.com/q/hashestowin)
Current Bitcoin reward per block: 25 (http://blockexplorer.com/q/bcperblock)

So it will remain profitable (i.e. value of mined BTC exceeds electricity costs) until network hash rate is more than 100x today's hash rate at current prices.