Will forks and alternative block chains cause hyper-inflation of decentralized digital money?
It depends entirely how much those alternate block-chains get utilized. In the recent examples of ixcoin and i0coin, a small portion of btc miners and users switched over, sold their stash and went right back to bitcoin. It's essentially a case of network effect - whichever block-chain has the most users will be the most attractive to new users, and those new users themselves contribute to that effect. Bitcoin is by no means beyond the "tipping point" where forks can no longer threaten it, but short of a fork with some superior feature or another it remains the preferred fork and probably will for some time. Even namecoin hasn't touched bitcoin's user-base much despite having a definitive use built into the protocol.