Well, they haven't taken "credit" yet (your description didn't mention them telling your husband that they got it... they haven't, have they?), and I think whats the most important is that he got a gift he's been wanting a long time- not who gave it to him.
Honestly, I think it is SLIGHTLY improper to ask for someone (or multiple people- even if family) to get your husband a gift... It is different when/if they ASK you what you suggest. I've NEVER suggested anyone to get my husband or kids something for their birthday or Christmas, etc... I only say something if someone asks. I think it is improper etiquette. Even though you were trying to split the cost, it still sounds very presumptuous of you.
On the other hand, it would have been polite of them to respond a yes or no to the emails. What it sounds like to me is that they didn't want to get the gift, but after finding out that you already purchased the expensive gift they possibly felt bad and wanted to lighten your load on the cost- which, in the end is kinda nice. It IS annoying that they keep "forgetting" the check. I'm assuming they are kinda old however, seeing as they are your husband's grandparents.
I would just not mention it to the grandparents anymore or your husband. If you eventually get the money, I'd still thank them. The last thing I'd do was to get mad. I'd get a bit irritated at worst. I'd also probably count this as a "senior moment" of his grandparents.
I and my husband are going to be 30 soon (me, in two months- him in 3 months). Although 30 is a cool milestone, I personally don't expect my family or his family to make a bigger deal of it than any other birthday. It seems by that age it is more of a "friend" celebration... but that's just my perspective.