Here's more from Dana Stevens in Slate about the presentation of abortion in Knocked Up. I won't insult Stevens by trying to outline her arguments, but in response I'll just point out that writer/director Judd Apatow seems to me to be paying a compliment to Katherine Heigl's Alison character by allowing her to attempt both motherhood and and a career. Making that difficult choice feels a lot more in tune with where we are as a culture at the moment. Would anyone want to see a movie about Alison's mother (played by Joanna Kerns), who (it is strongly implied) has sacrificed some level of personal happiness to raise her children?
Another point: Alison's brother-in-law (Paul Rudd) mentions that he married Alison's sister (Leslie Mann) due to a pregnancy, which means Alison's family has dealt with this issue before. We don't get a great deal of information about the dynamics of Alison's family, but it's reasonable to assume that Alison is modeling her choices on those already faced by her sister.