Diapers are a wonderful idea. When I was expecting, I bought one pack of diapers every time I went to the store . . . and by the time the baby arrived, I had a closet full. Brand and size wasn't really an issue. Since I didn't have a paycheck for a while, those pre-purchased diapers were a great help.
Then I discovered a fantastic local woman (mother of two sets of twins) who sold second-quality diapers and delivered them right to your house. Very cheap, and I stored the large boxes under the crib.
If I were trying to buy diapers for someone else, I think I'd go with automatic delivery from
Amazon. Diapers could be delivered straight to her door every month (or every other week), and if she discovered a problem with a brand (which I suspect is pretty rare), she could ask you to change the details of the order.
A breastfeeding pillow is a great gift. A new mother will spend hours feeding the baby, and that pillow is exactly the right shape to make the job easier -- it's a huge back-saver for mom. Likewise, gowns with hidden splits down the front are a nice gift for mom.
I also like the idea of books. This is genuinely a gift that "keeps on giving" in that it enhances the child's mind. Choose tough, chew-able board books.
I have frequently given a gift card to a photo studio -- especially for a second baby, when the mom already has everything. If you know that the mom doesn't have a nice point-and-shoot camera, that is even better -- but you'd know whether your friend does or doesn't have one already.
As for car seats, the concern about "expiring" is really overstated. Keep in mind that by the time the 5-6 year point is reached, the child is probably at the booster seat stage -- and that means that the seat is just "boosting" the kid up high enough to use the car's built-in seat belt. That doesn't "expire". What's more important is that the car seat has never been in a wreck.
Having said that, I totally agree that no one wants to use the same car seat for 10 years. Those things get nasty, even if your cover is washable.
Finally, several people have suggested Graco this or that. I vote NO. I had a moderately expensive high chair from Graco, and it was great for my first child. We put it away for a while in the back of our coat closet . . . and when we pulled it out for our second, the plastic was split in several places. We ended up buying a cloth cover that never really fit well. We also bought a Graco Lite-something stroller, and the wheels "melted" and "warped" the very first time it was left in the trunk of the car (my children were summer babies, and we live in the South, but you'd expect to be able to keep your stroller in the trunk of the car). I called and asked for new wheels, and they told me that what I was describing was impossible -- they DO NOT stand behind their products.