Yes, I knew, and I had also read in guidebooks that if you approach Barcelonians and start speaking to them in Castellano (Spanish, but they prefer calling it Castellano), they'll give you dirty looks. Thankfully, the latter turned out to be quite false, as most of the people I've talked to have turned out to be very helpful. However, Catalán is very widespread, and it does seem to be the language of choice.
All of the metro signs are in Catalán, as are most of the advertisements and billboards. About half of the classes offered at the Universitat de Barcelona (UB) are in Catalán. It's also very rare for two people who are both from Barcelona (or all of Catalunya, for that matter) to be caught speaking only Spanish to one another. The preferred language of communication is a mix of Spanish and Catalán. And by mix I mean one person will ask a question in Catalán, and the other will respond in Castellano, for example. The languages aren't really meshed together to create a "new" language like "Spanglish" has done by mixing Spanish and English words; and this probably because all Barcelonians grow up bi-lingual, while Spanish-speaking people who come to America, especially older ones, initially have a heard time learning English. So, there was no need to create a new form of communication that brought together Spanish and Catalán.
The omnipresence of Catalán makes things slightly more difficult, especially when it comes to hanging around other Spanish students and trying to understand what they're talking about. Because the two languages are so similar, someone who speaks Spanish will have little difficulty understanding written Catalán - but of course the spoken language is much harder to follow and comprehend.
At my orientation, I was forewarned that some of my professors might randomly start speaking in Catalán during a lecture (even though the class language is stated as Castellano). I'll soon find out if there's any truth to that statement when I start my classes at the UB on Monday.