The vice-president of the European Commission, Joaquin Almunia (who previously worked for the PSOE) decided to wade into the separatism debate and say that if Catalunya separates from Spain, it would be out of the EU.
There’s a couple flaws in that argument, the most important of which presupposes that Catalunya would actually be first recognized as an independent country by anyone. If Catalunya isn’t recognized as a sovereign country, it can’t be kicked out of the EU. The idea that the central government in Spain, the rest of the EU and the UN Security Council would unanimously agree on splitting up Spain is unlikely in the extreme.
So what would happen if Artur Mas declared Catalunya independent? Basically nothing, at least until Catalunya decides to start collecting taxes and social security on its own (which is the only thing this is really about). At that point, it would be difficult for the central government to do much except take the Generalitat to court. Most likely the Generalitat will lose, but then who will enforce the will of the court? The Mossos? The National Police? The army?
Financially it would be chaos at that point, since no one would know who they have to pay their taxes to. If they pay to the central government, they might still owe to Catalunya and vice version.
The most reaction from the rest of the world would be to ignore any declaration of independence and tell Spain to get its own house in order and figure out some political compromise that would allow the requisite amount of independence without needing to create a new state. It’s not like this hasn’t already been done with Navarra and the Basque region.