Oh, because I've done some research on it, I'll state the two unnoticed possibilities:
Albania.
That's right; that small Muslim country in the Adriatic. Back in the early '90's, they really, really loved the United States. Had the United States been receptive to the incipient annexation movements back then, it is quite possible that Albania could be the 51st state. That, of course, requires several major POD's to get America receptive to such a possibility, but it is not utterly inconceivable.
The other one:
Taiwan
Assume the Third Taiwan Straits Crisis goes badly and the United States and China duke it out (a distinct possibility; cf. Taiwan Relations Act and Formosa Resolution). Assuming the war would stay non-nuclear (another likelihood), then post bellum, Taiwan will need to solidify its position across the straits. Hence, annexation is proposed (assuming the Taiwanese realize that the United States may not always want to defend the island) and, assuming US anger at China is sufficient, we agree to annex the island (in fact, there may exist some legal justification for doing this right now under the Treaty of San Francisco). Statehood roles around sometime in the early to mid 2000's; a little past the time frame specified, but whatever.
Both of these are long odds, but again, not entirely inconceivable; after all, the Taiwanese economy is fairly large for its population.
And now for shameless self-promotion: If you want to see Puerto Rico, Taiwan, and Albania as US states check out my sig.