Are bitcoins really immune to inflation?
Specifically, bitcoins are immune to M0/MB inflation, meaning that the money supply itself does not inflate, except at the very beginning (which we're still in) while the original 21 million BTC get distributed via the mining process. Once 21 million coins exist, they become deflationary since no new coins are issued and, as naturally occurs, money falls out of circulation as wallets are lost.

Bitcoin could still suffer the kinds of inflation most currencies see in M1/M2/M3/MZM such as fractional-reserve banking, but the idea of a publicly published block chain is that at any moment it should be trivial to audit a bank and ensure their deposits are correctly recorded. Essentially, any institution practicing fractional reserve banking should be much easier to spot and those who dislike such practices can more easily walk away from them. There are complications to this model, but that's the ideal anyway.