Ok, the reason why they are cautioning you is because what you eat/ take does go into your breast milk.  Breastfed babies generally don't have a problem pooing as it is, so adding a laxative to the mix could upset that balance.

You could try drinking some of your own breast milk, which is considered to be a natural laxative.

if you want to use a chemical laxative it's best to heed the warnings, and a doctor will be able to tell you if you can or can't, and what the contraindications are.

i'd suggest trying some natural laxatives, like prunes and liquorice, a large number of dried apricots seems to work for me :P

There is a medicines information line for pregnant and breastfeeding mothers and I can't remember the name of it for the life of me, and I can't find the number, but if you can find it - any pharmacist should know it - they'd be able to tell you over the phone whether or not it'll be okay to take.

Things like laxatives are such a common thing that there are so many different types that the effects can vary widely, so I'd strongly recommend finding out more about that particular brand before taking them.

Usually when a medication has that warning it means that insufficient testing has been done to fully prove it safe while pregnant or breastfeeding. It might mean that it will generally be okay in small doses occassionally, but at high doses or for a long period of time it might have negative effects, or that they just don't know yet if the chemicals from the drug that pass through the breastmilk or placenta might have a non-obvious effect on the baby (eg: delayed development of some sort further down the track), or on you while your body is trying to support the baby (eg: you're body may already be struggling to keep up calcium supplies and the medication might deplete it more). The warning is a safety blanket effect to protect the company from being sued by someone that didn't seek medical advice for any reason if there is even the slightest risk of a negative effect that the company wasn't fully aware could happen.