The validity of transactions is checked by the individual bitcoin clients. After all, something, somewhere must check if they are valid according to the protocol rules! And this client code can be changed to anything at all, as long as most miners agree to update to it.
Transactions that make up bitcoin out of nothing are already used as rewards for mining a block.
Individual clients don't have to use the same client that miners use.
If 51% of miners start using an inflationary version of Bitcoin "Bitcoin-QE", but the clients (exchanges, merchants, wallet providers, etc) don't approve, they won't update the client and use the partition of the network that is secured by the remaining 49% of miners. People wouldn't want to accept the Bitcoin-QE mined on the inflationary branch and the investment wouldn't pay off.
Of course Bitcoin-QE miners could start an attack on Bitcoin in other ways, but that also woudn't be necessarily economical.
It's very hard to tell what 51% will do, but it is clear that you don't have full control of the currency.
Transactions that make up bitcoin out of nothing are already used as rewards for mining a block.