We have no data on Tamiflu in breastfeeding mothers. If you look at the efficacy of this product in reducing symptoms and severity of flu, it is only marginal, perhaps only reducing the duration by a day at best and then ONLY if you take it immediately when symptoms appear.
This is one of those cases where in most normal women... taking the medication is not worth the risk (however low) to the infant.
Basically, if you are going to gain a lot from taking a medication, then it might be worth a little risk to the infant. But if you're NOT going to gain much...then I wouldn't take it.