I have an Acme Made case which is really cute and fits like a glove over my laptop. My bag is the Designer Slim, and it has carrying handles and a detachable shoulder strap. http://www.acmemade.com/thedesignerslim.html
I also have an Acme Made case...there is an amazing choice of deisgner fabrics. I have the Plural Fossil and I love it....so beautiful and my laptop fits perfectly. http://www.acmemade.com/thedesignerslim.html
Wow, the Acme Made cases are SO cute -- I have been using an Incase sleeve and putting that in my everyday work bag. It might be time for an upgrade and purchase one of the cute Acme's.
I put a foofbag sleeve on mine and then put it in a purse. I have the tiny 12" Apple PowerBook, so it pretty much fits in anything. I'm a big fan of sleeves in general, since they protect the laptop and don't take up much space.
^i would recommend getting a laptop bag rather than using a normal handbag since extra space causes the laptop to fall around and can damage the hardware. laptop cases are designed so that the laptops are cushioned.