I would second the opinion to check out the Queer Parenting forum. There are lots of lesbians and gay men parenting together who aren't romantically involved. I don't think most of them live together, however. Most of the successful situations that I'm aware of have involved specific and detailed written agreements. Often, the relationships have to be renegotiated as the child grows. It seems that the key is really communication and a clear understanding of roles and responsibilities.
If you are both going to function as co-parents, I think it's important to make it as legal as possible, for the child's best interest and well-being. That means that his or her relationship with you will not be severed if your friend decides to move to another state or has an irreconcilable conflict with you, or whatever. As someone else noted, parenting is incredibly stressful, and 18 years is a long time. It also covers your child's rights in terms of inheritance, social security benefits, etc. should something happen to you.
You need to think and talk through the financial implications (who's going to pay for preschool? college? clothing? who takes the tax deductions?), how is the child going to be connected (or not) to your biological family, what will happen if and when one or both of you become romantically involved with someone else, what if you or her choose to give birth to another child later, etc. How are you going to make decisions such as in what religion will the child be raised? Where will the child go to school? Where will he or she spend holidays?
And then, there's making sure you're on the same page about your parenting and child-rearing beliefs. It's always good to talk through these things before the baby is born, but even more so when you are bringing a child into a relationship that is not supported by society and by legal and social structures.
Finally, are you comfortable being identified as a lesbian couple raising a kid? Because that's what two women equally co-parenting is going to look like to others. Where I live, it's no big deal, but you might want to think through this in advance and make sure you feel comfortable with it.
Best wishes -