Not entirely impractically if you're the US Govt, but very very very expensive 
Estimates are around $666m right now, to do a 51% attack [1].
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[1] http://www.coinometrics.com/bitcoin/rix
Estimates are around $666m right now, to do a 51% attack [1].---
[1] http://www.coinometrics.com/bitcoin/rix
You could do a 51% attack with a subsidized mining pool and the greedy ass miners.
It would be far cheaper than $666 million
