First of all, J, it's great (and a relief) that you're able to leave this job under circumstances that are neither desperate nor disgruntled. You're actually in a great position to leave your firm. Your approach with your firm and your manager is straightforward and polite:
"I wanted to let you know that my husband has accepted a position out of state. We're both really excited about this for him, but unfortunately it means I have to leave this firm. I've really enjoyed working here with such great, talented people and have learned so much. I'd like to work here as long as I can before we move in June, and I'm glad to train anyone new that you bring on board before I leave. I'd also like to make sure that I can rely on you for a good reference--if you're comfortable doing so--when I apply to other firms in our new location."
Interns and architects leave firms because of a spouse's job all the time. It's a reason to leave a firm that generally makes for no hard feelings, so I see little to no reason to worry about a graceful exit from this firm.