I did the opposite... brought a 5 month old puppy in about 2 weeks ago into my house with two 3 year old cats. It's been a work in progress, but something definitely clicked this weekend, and all the sudden they get along great.
In fact, this morning, I'm sitting on the couch sipping coffee, and the dog (30+ lb german shepherd / malinois mix) comes tear-assing around the corner with a cat hot on her tail. It was great to see them playing.
I set up a very large dog door with a passthrough right outside my room. For the first few days the cats wouldn't come out. Then they started inching out, and at first the dog immediately tried to chase them and scared them back in. Now, the dog responds to me saying "easy" and goes up slowly, sniffs, licks, etc. It gets better and better every day.