I know I'll be flamed by ever US/UK shipper out there, but it will not happen. The only way this pairing can become canon is if Himaruya directly tells the fans that they will get together, whether it's in a strip or an comment told directly to his fans. And no, Funi does not count. They're teasing you guys.
LOLOLOL Can I tell you how much I love you, Lilly?
Anyway, just wanted to add that the probability of -any- number of countries being paired together in any way NOT established early-on by Himaruya is very low. Three published manga and dozens of episodes have only given us a single kiss--that between Chibitalia and the Holy Roman Empire (who 'died' soon thereafter). And while pairings like Austria/Hungary and Germany/Italy (this one's a little convoluted) are -hinted- at, the possible romances in Hetalia seem to be mostly fanservice and hence up to fanon.
As becoming a couple or 'getting married' appears to be symbolized primarily by two or more countries forming a union (e.g. Polish-Lithuanian Commonwealth)/signing a unifying treaty (or something similar), and it's highly improbable that the United Kingdom and the United States will ever have to do either/or (with one another, at least) in the near future, it doesn't seem like Joker will be happening in canon. Although, even if two or more countries do the aforementioned, it doesn't necessarily mean that romance will happen, either.
On the upside, Himaruya -has- hinted that he has a favorite couple (if I remember correctly; correct me if I'm wrong, please), and while I'm thinking it's probably Germany/Italy, the ambiguity of England and America's relationship IN THE SERIES can make one wonder if he ships the two, himself.
...then again, fandom pretty much does what it likes in the end, no? *coming from a FrUK shipper, jsyk*
Also, I might be wrong, but it feels like there was a minor increase in the number of France and England strips (of their past) over 2009-2010, so who knows? Maybe Himaruya debates the Blond Triangle, too.