Hello Robbie,
So far, though sigma has
announced it, they have not released any price info or the availability date. However, I would be surprised if it was much different from their
30mm 1.4 hsm lens that also autofocuses with the D40 ($364 @ sigma4less). As far as availability, I would be surprised if it was more than 3 months.
Given that, who knows when nikon will get around to releasing a 50mm AF-S lens.. people have been clamoring one for years and there hasn't been a hint.
Like you said, there is always the AF 50mm 1.8 nikkor. Yes, you will have to MF it on the your D40, but to clarify what Chris said, the E-series of this lens will not meter, but the AF-D 50mm 1.8 will meter. You can find this lens for around $100. But, he is right in that you could probably sell the lens used for about as much you paid for it later on.
It's a fun lens. My wife got me a copy for xmas last year. I've been getting used to MF-ing, but sometimes I wish I did have autofocus. But then I ask myself, is it really worth ~$250 just to be able to autofocus.
To each their own. If it's worth it to you, then I'd just wait for the sigma. It can't be that long till it comes out.
Good luck, and have fun.
DCBass