Will transaction fees eventually make microtransactions not worthwhile? It seems that one of the strengths of Bitcoin is the ability to send tiny amounts across the globe. If we keep thinking in USD or EUR, we'll wipe out micro transactions altogether (less than 100 satoshi).

So will transaction fees eventually put a lower bound on how small of a transaction can be sent?
A change to the 500KB per-block limit can be done with updating the client without causing a fork, but that only goes up to a 1M block size.  To exceed that 1M limit would require a hard fork and thus it is possible when that limit approached, a larger limit might never happen.

Bitcoin could hit those limits before really even "taking off" even.

Bitcoin was never architected to be a microtransaction system.  It works fine as one for now but eventually that use might no longer be feasible.

There are currently methods to use bitcoins for microtransactions without touching the blockchain.  For example, Mt. Gox BTC redeemable codes have no fee (currently) and could be used, presumably, for microtransactions.  Various approaches will solve the microtransactions problem better than Bitcoin does.

In the meantime, businesses that do use microtransactions can get off the ground by taking advantage of the fact that Bitcoin's is still well below the per-block capacity limit and thus microtransactions are nearly free.

Build a customer base in a market today, solve the technical problems tomorrow.

With Satoshi Dice's growth, for instance, there might soon be enough demand to launch a alternate blockchain for its microtransactions with properties that are useful for that special case.