Well, now that I'm finally registered I can give my 2 cents.
Imo, I'd rather they did NOT do it than having them have sex drunk/intoxicated/with memory impairment. I do not think that's what Amy is meant to represent. She's not just a plot device to get him to finally succumb to lust just for laughs, I actually think there's a much deeper meaning in their relationship, with her being that one-of-a-kind woman who's of his own species. The exception to the rule, if you will. So if they do decide to do it, and I think it is very likely, it will be too monumental in terms of character growth to be cheapened for laughs. Even though it is a sitcom, they can still recognize when they need to be serious about stuff (think of Howard's apology to Bernadette last season). I do think however that it won't happen for a long time, although I do not necessarily think they need to wait until the very last episode: there's SO much comedy potential when these two finally do it that it would be a waste to leave it to the very end. I can only imagine how you could play it out with them, especially Sheldon, hiding it from the group, being teased by their friends, being caught red-handed etc. So much comedy gold right there. So I'm calling it some time between end of the penultimate and mid-way through the final season.
I also disagree with those who say that Sheldon having sex wouldn't be Sheldon anymore. First of all, I think there's more to what makes Sheldon Sheldon than his virginity. And secondly, I think it depends. Sheldon suddenly turning full-on heterosexual man and having sex with anyone would be out of character. But, again, this is Amy we're talking about. I really agree with Bill Prady (I think it was) when he said that this character is capable of things we haven't seen yet. We haven't seen them because he's never been put in the right situation: he has never met someone who he considers to be his intellectual equal, who he is not uncomfortable with and who he has an emotional attachment for. He isn't "changing" with everyone else, because everyone else isn't Amy. So it doesn't make sense to say "Sheldon would never do that", because this situation is one of a kind, so how Sheldon would function in the world outside this situation has only some relevance to how he functions with her. So going back to sex, I do think they could play the card of "He is just asexual, end of the story" and never have them have sex, and that would be reasonable and in-character. But I think there's a second interpretation for his lack of interest in the opposite (or same for that matter) sex: a ) he jumped straight from childhood to adulthood, so he never experienced a normal adolescence and b ) he never met anyone who he considers to be of his same "species". In this framework, it could very well lead to him sleeping with Amy eventually and at the same time be in character. All these jokes about him hitting puberty, Penny asking him about fuzz in weird places, him getting a testosterone level etc... I think they are actually a very accurate representation for what's going on.
In any case, whether they do do it or not, I think it's a sad state of affair when I hear people put their relationship in inverted commas just because they aren't physical. As if what makes a relationship "real" is whether sex is involved. I think their relationship is much more real than many others, fictional and non-fictional, and tip my hat to that. I really hope the writers keep up the good work with them, they have done an excellent job so far.