We have a few free days in Europe after we attend two (!) weddings in Slovakia and I've always wanted to see Barcelona. We're free starting August 31 through September 5. Is that a good time to go? What is the weather like? Are shops closed due to vacations? Or should we consider another location?

Unfortunately, no one will know what will happen in September 2010.
In 2008, the first week of September was very nice. Always sunny, daytime temps around 30C, nighttime temps around 25C, water temps above 20C. I stayed 100 meters from the beaches, so I'd often break the day from 2-4pm to return to the apartment, and cool off in the sea. Lots of tourists in town.
I did not notice any shops being closed for vacation.

Weather and statistics are not the same science so you can look at the past figures but they will never determine the future.
http://www.aemet.es/en/-m:b,a/elclim...s?l=0076&k=cat

Hot and humid probably but still very MUCH worth seeing IMO. I've done the city several times in August: there will be lots of visitors and plenty of places, including restaurants, very much open for business. I even saw an opera performance in the stunning opera house in August and it was jammed.

Booking a hotel with a pool might be a good idea.

We've been there about a month later than this and it was delightful. Almost cool some evenings, up to 80F days, ocean water was very comfortable. Everything should be open.

Make sure you're apartment or hotel is air conditioned - most are but check the fine print.

Thank you all - this is very helpful.