Oh, Yikes! Tacky, tacky! Partly bad since you don't socialize with them on a regular casual basis, but ESPECIALLY b/c as your DH's client, they're in a bit of a position of power over you. How do you say, "stuff it"..errr, I mean, "no, thank you; we won't be attending." <---- Emily Post probably wouldn't agree with that first response.

Nice or not, some people really need to get print outs of these etiquette threads.
Though on the flip side, you could look at it as they like you guys SO MUCH, they're treating you the same way they would one of their close friends from whom they would solicit hot dogs and maybe some tequila. It's like a compliment