This is a really interesting discussion as the concept of a virtual economy is different from a real one.
Both the "cash shop gems/PLEX" and gold are currencies, as they can be used to purchase goods and services.
Purchasing gold with PLEX doesn't cause the price of goods to inflate, it causes gold to appreciate in terms of PLEX. In a graph like this one:
http://hsc.csu.edu.au/economics/place/exchange_rates/figure1.gif
, and increase in demand for gold would cause a rightward shift on the demand curve, increasing its price in terms of PLEX.
However, people who buy gold may be more inclined to spent it recklessly, and that could be considered as a determinant of aggregate demand (aggregate demand is all the spending that happens in an economy). As there is more spending, there's a rightward shift on aggregate demand, which raises the price level of goods (A raise in the price level means there's inflation).
(In case you're wondering, this is what the graph looks like: http://stephenkinsella.net/WordPress/wp-content/uploads/2007/02/aggregate_d emand.JPG )
As long as you can't directly exchange PLEX at a fixed rate for currency the server creates when you pay for it, I don't think inflation because of PLEX is a problem, as gold is still increasing at a rate that is unaffected by the purchase of PLEX.
In a slight tangent, I'd like to talk about why inflation would be bad.
One problem with inflation is the redistribution effect, which happens when people who gain fixed incomes have less purchasing power, as everything costs more because of an increase in the price level of goods and services.
However, most people in this virtual economy gain both fixed incomes by the gold they get from monsters and NPC quest rewards (if there's going to be any NPC quests), and incomes that would scale with inflation by what they produce (crafting/gathering skills and gear they get from monsters), as they can sell them at a higher price.
With high rates of inflation, the gold we get from NPCs would progressively become insignificant. However, if that's the only injection of money to the economy, then the leakages to the economy should cancel them out as long as they're well thought out (and with full knowledge of all the money coming in and out then the devs should be able to balance it to some degree of accuracy).
But if the ability to exchange PLEX at a fixed exchange rate for gold is created, then I could see that making the amount of gold we get from adventuring become insignificant.
I'm not saying all my conclusions are right, but hopefully this will contribute to the discussion.