Well, I haven't actually done it yet, but after reading a lot of MDC threads and doing some other research we're hoping to have another baby (birth, not conception) when DD's a few months shy of four.
I never intended to space my kids this far apart, but:
- DD is currently 2 1/3. I can NOT imagine having a newborn to care for as well as her! She's not a "hard" toddler, but she still breastfeeds, including at night, cosleeps and is not toilet trained (the latter is probably because I'm lazy, not because she couldn't do it, but still). She's not ready to nightwean or sleep alone, and I don't fancy the thought of sleeping with a baby AND a toddler - and a husband - in a mere queen-sized bed.
- I'll tandem-nurse if I have to, but I'd like DD to be old enough that if I feel the need to wean during pregnancy, or if my milk dries up, it won't be such a disaster. I think it's 70% of women who lose their milk supply during pregnancy? I'd like to CLW but if we start TTC when DD turns 3, I won't feel too guilty abut weaning if I have to.
- I've been reading a lot of Traditional Foodsy stuff lately, and one thing that kept cropping up was that traditional cultures tend to space children to 4ish years, to enable the mother to recover her nutrient stores. My diet is OK but not great, and I'd like to do some aggressive nutrition before I TTC again. I'm the third closely-spaced child in my family (3 kids in under 4 years) and think I can see the results of Mum's nutrient depletion in her health and mine.
- I'm only 24 - had DD when I was 21 - so I still have plenty of time, in theory, to have several children with generous spacing.
- I love the idea of DD being a help rather than a hindrance when the new baby comes, being really curious and interested in the process of pregnancy and birth, etc.
Plus there are some shallower reasons - like, it's easier to ask people to babysit a baby and preschooler rather than a baby and toddler! And, in all honesty, we might TTC a few months sooner except that we're taking an overseas trip next March. And Disneyland doesn't let pregnant women ride the coasters.

I do feel twinges of guilt that DD won't have a super-close-in-age sibling, but then, in my experience it evens out in the end. I mosly fought with my close-in-age siblings growing up; now as a 24-year-old, I'm close with all of them, from the 28-year-old to the 15-year-old and 13-year-old. If I were starting my family in my late thirties I'd definitely space them closer, but as it is - I love DD dearly, but I find the whole process of pregnancy, birth, postpartum recovery etc very taxing and I like having a bit of space to just be me between babies.
That said, I'll let you know how it turns out.

Could be a disaster!

And it definitely puts more pressure on us to TTC "efficiently" - I'd rather not have kids 5 or 6 years apart, so I'd start to feel pretty stressed if we didn't successfully conceive within the first few months of trying. As far as I know I have no fertility issues and I do chart, which increases our chances of conceiving early - but, you know. Something else to consider...