don't want to be negative but Brenthaven doesn't quite fit that bill IMHO, case in point - it was purchased for your mother-in-law.
I have a timbuk2 bag with a sleve.
http://www.timbuk2.com/tb2/retail/catalog.htm?categoryId=6&skusetId=85
Just posted this about a 12" but they say it fits the 15 also. I can attest to the fack that it is a super cool bag, that not everyone has. The laptop sleeve comes out and the shoulder strap attaches to it so you can carry just your computer and leave the full bag back in the hotel room. Kinda cool http://www.groundwear.com/products.php?style=62
I recommend Crumpler for uber cools bags. They're quality fabrics, great colours and will out last many of the bags on the market today.