Everytime I have out-of-town guests, I am asked the same question. Do most people in Puerto Rico speak English? Is it okay if I don't speak any Spanish at all? The answer is.... it really depends. If you just stick to the tourist areas, yes, you don't have to mutter a single word of Spanish. Although knowing how to say "thank you," gracias, would surely be nice. For non-Spanish speakers, I think you can definitely survive here. However, you might have a bit more trouble once you leave the metropolitan area of San Juan.
Outside of the resorts and hotels, I would say you'll find more people who might have a limited knowledge of English. However, practically everyone speaks a little. Remember, we do speak Spanglish here and the Island of Enchantment is officially part of the U.S. In reality, Puerto Rico is heavily influenced by the U.S. Just walk through the island's largest mall, Plaza Las Américas, and you'll understand what I mean. You'll find Macy's, Banana Republic, Pottery Barn, etc. If you're from the States, you'll actually feel quite at home. Best Buy, Wal-Mart, K-Mart (Big K too!), and Home Depot, just to name a few, are also in Puerto Rico. At the supermarkets, you'll find your favorite brands such as Heinz Ketchup, Oscar Meyer, Pepperidge Farm products, amongst many others. This is no surprise, as Puerto Rico manufactures and grows very little of its own food products. Practically everything here is imported from the mainland. My only beef is that we don't have Target here. If they do ever decide to come to Puerto Rico, I am all set!