There's a debate going on down in the Comments section, and it's spilled over into Trevor's blog Kaleboel.
(Scroll down--you may have to scroll down a long way--to about the fifth post, titled Spanish / Castilian. Trevor, when I enter your blog, I get the "header" and then blank space about 3/4 the way down the scroll, if you see what I mean. Is this just my computer?)
Here's the official Iberian Notes Press Release on the topic.
1. In English the language spoken in Spain and most of Latin America is called "Spanish". "Castilian" refers to the dialect of Spanish spoken in the northern two-thirds of Spain; it's characterized by the "lisping" TH sound for Z and C before E or I. "Peninsular" refers to features common to all dialects of Spanish in Spain--for example, the use of "vosotros". This is English and those are the rules we use. It personally doesn't matter to me whether you or I say "espanol" or "castellano" when speaking Spanish. I think they are interchangeable. When speaking Catalan I say "castella", of course, that being the standard term in Catalan. You can say what you want as long as I can understand what you say.
(Note: Same deal works for "American". In English it means a person from the United States. [Also in French, German, Italian, and Russian.] Some people claim that in Spanish "americano" refers to anyone from the Americas. Fair enough, in Spanish. If the rule is that what we call "American", you call "estadounidense" or "norteamericano", then fine, when we're speaking Spanish that's the terminology we will use. But when we're speaking English I will use the right word in English. You can do whatever you want.)
2. I think that, absent government regulations, the language spoken in a place ought to respond to the market. That is, you will use the language(s) you need to use to get along. In that case, most people will speak both Catalan and Spanish, here in Barcelona. If you work at a supermarket, for example, some of the customers are going to want to use one and some are going to want to use the other, so it's perfectly acceptable for the boss to demand that the worker be basically competent in both languages, at least to greet clients, make change, and say goodbye.
However, there's something that Catalan nationalists refuse to accept, which is that people have the right to live here and not learn Catalan if they don't want to or need to. If I'm the client, you should adapt to my language. And if I'm the boss, you should adapt to my language. And if that language is Spanish rather than Catalan, or vice versa, then you'll just have to put up with it. See what I mean? It's not that hard.
For international purposes, Catalan has little appeal except for those who are interested in Catalonia, its culture, and its language. (Valuable things to be interested in. I am interested in them. That is why I live here.) They will want to learn Catalan. The rest will want to deal in Spanish or in English, whichever language they already know. You can't force foreigners to use Catalan, nor can you force people from the rest of Spain to do so.
Here's an example. Say you are half liquored-up at some party and it's two in the morning, and you are flirting with someone attractive. You both REALLY want to communicate to one another that your genitals are becoming inflamed. What language are you going to use? Let's say I speak English best, then Spanish, then Catalan, then French. You speak Catalan best, then Spanish, then French, and then English. Which language are we going to use? Now be honest.