We've been thinking about this. I actually went to a preconception visit with my ob/gyn about it (we had problems before dd). DD is 9 mos. old.
The ob said that the medical party line was 9 mos between birth and a conception was what they 'like to see' and 6 months was an acceptable minimum with a c-birth or otherwise in his opinion. Physically your body can handle 6 mos. between in most cases.
If you're nursing, also, you may not be ovulating. this dr. said that with 28 day cycles and breastfeeding you likely were (we're still having 35 day cycles which he said were most likely not ovulatory... or at least that's what i think he said)
He said that your milk supply does tend to decrease when you're around 4 months pregnant and some people don't nurse after that, but i know several people who tandem and nursed through a pregnancy, so I would maybe take that advice a little less seriously. Just something to think about if you're planning to nurse your current little one until age 2 or more... it can be done, but I do shudder when I remember how the first 3 months of pregnancy wiped me out. Then again, I'm an older mom.
Emotionally, and this is my own personal deal which may be different for you, i WANT two children very close in age.. my own age is also a factor, as is the previous problems we had.
I don't think there's a universal good or bad spacing, it's just what you want and what will work; the physical limitations aren't necessarily set in stone either, but if you wait till 10 months, you're good with those, anyway.