First, the Apple Store has a lousy selection of laptop bags/cases/sleeves. You will do better finding your own. Once upon a time, they used to list a lot of them, now they only list those with whom ethey have kickback payment arrangements, which dumped a lot of companies like Timkuk2 and Crumpler who have always been their strongest supporters.
I am going to pipe in on this one being a total bag addict and not finding one that really suits all purposes, so I have several...but....
First, I love the be.ez sleeve; it is made of hard supported foam core and not neoprene, and it protects better and doesn't have that awful burnt rubber smell which not only irritates me beyond all getout, but also makes your bags smell (and anything else you put into them).
Second, I have found absolutely nothing better than the Timbuk2 Commute Laptop messenger bags. Do NOT get the standard laptop messenger bag. Make sure it is a TImbuk2 Commute -- it has many pockets, many of them zip, and it is lightweight and fits about anything you can put into it. Their standard laptop messenger bag does NOT have enough pockets, and the shape is weird.
Third -- the best backpacks I have found are from Oakley. They designed them with Apple portables in mind, and they have great protection -- but they also have large roomy storage areas, and are even big enough, for example, to put a dSLR camera in the main compartment along with your laptop if you ever need to down the road, completely eliminating the need for those impractical camera/SLR bags from Lowepro and the like. Booq also has good, super-protective bags -- but I think they are overkill -- they have strange shapes, and they are heavy.
I, for one among others, am NOT an InCase fan -- They are nothing more than sleeves with exterior pockets -- and I don't know where all the folks posting live, but if you are in a rainy/snowy/dusty climate, they are completely inappropriate. While the computer section does close off with a zipper, the external pockets under the flap are exposed to rain if you have anything inside them. Not good design for anyone in environments that are not dry and dust free all year round.
Another great option, but expensive, are Waterfield Bags (SFbags.com). They make great, strong bags, and their service department couldn't be better. Their sleeves are second to none - and you can get them with straps and flaps and piggy-back attachments. Their Cargo messengers are just the best bags you can get -- although they become expensive since you need to get a sleeve to use inside them. Depending on your formality needs, though, these are the most professional looking bags you can get that are still sporty and fun. I like the airplane seat buckle that they use, and they also have a paraglider buckle in their newer styling. Not cheap though. And they are relatively heavy.
My personal recommendation for you based on your posts -- the Timbuk2 Commute Messenger in Medium. Since you want it to be more "formal" get the one that is all black. It's 100.00 at Timbuk2.com or at eBags.com or possibly near you if you have a bicycle store near you, or outdoor store (REI carries them). They occasionally go on sale. You can also find them on eBay since they are popular items and can probably get a black medium one for about 70.00 -- although on eBay, you will be getting last years model, not the newer one with the back that folds down.
