There are seemingly a countless number of bag choices, but I thought I'd offer some of my favorites. For a good-looking bag that isn't out of place in a business environment I highly recommend the Cosmo bag from Waterfield Design ($149-$169). For a more sporty messenger style bag for students, I'd recommend a customized bag from Timbuk2 ($75-$150).
Whichever way you go on a bag you'll probably also want to have a sleevecase to give the MacBook (Pro) more protection in the bag. Waterfield has a nice model, but there are others available from the likes of booq, Brenthaven and many others (~$30-$60).
A more recent trend for MacBook (Pro) protection is the SeeThru plastic shell from Speck ($50), which snaps onto your Mac portable. The advantage of this style is that it remains on the computer during regular use and so provides some protection against knocks and scuffs. By the same token, however, it adds some bulk and negatively affects Apples elegant design.
External Mouse (and Keyboard?)
Some folks around here aren't particularly fond of the Mighty Mouse ($69), but the Bluetooth wireless version is a good travel companion considering that it can run on a single AA battery if need be. Another good option is the Logitech Revolution VX ($80) which includes a small USB transceiver dongle that slips into the mouse itself when not in use.
Although you might not generally think about, there are times when having an external keyboard can be a real boon, and the Apple wireless Keyboard ($79) is a pretty easy to carry travel companion (though you'll probably want a case as well).
Extra Battery or Power Adapter
An extra battery can be incredibly handy, and while not a sexy gift would certainly be appreciated (so falling into my category of good gifts that I might not buy for myself). Batteries for all of Apple's shipping lineup of portables (MacBook White, MB Black, MB Pro 15, MB Pro 17) are $129 each. Apple still sells batteries for older portables too, which may be even more useful as battery life goes down as the number of charge cycles increases.
A spare power adapter can also be quite convenient, especially if the computer is commonly used in two places (e.g. home and work). Apple sells a 60 watt MagSafe model for the MacBook and a 85 watt MagSafe model for the MacBook Pros for $79 each. Cheaper third-party adapters ($40) are available for older PowerBooks and iBooks that came before the proprietary MagSafe connector.
If your giftee uses a Mac portable at a desk, an external monitor can be a very nice accessory. Of course Apple sells Cinema Displays in 20 inch ($599), 23 inch ($899), and 30 inch ($1799) sizes (the 30" requires a MacBook Pro). However, you can find displays with similar performance for less money. Dell displays, for instance, sometimes use the same LCD panels as Apple displays and tend to be much cheaper (though, admittedly, uglier). Macworld has a nice set of reviews up on budget 20" displays that's definitely worth a look. Whichever display you get, however, make sure it has a DVI input (almost all do), and if you're a MacBook user you'll also need to pick up an Apple mini-DVI to DVI adapter ($19).
Another great tool for the desk-bound Mac portable users is a riser that lifts the display closer to eye level (and of course requires an external keyboard and mouse). I personally recommend the Griffin Elevator ($40).
For a more portable solution you might want to check out a portable lapdesk like the Lapinator ($25 or plus size, $30) or the Targus Notebook Portable LapDesk ($30). Either of these will help keep a hot Mac portable off the lap and save scalded thighs.
Portable Hard Drive
Especially with the advent of Time Machine in Leopard a portable hard drive becomes even more useful to the Mac user on the go. You'll want to go with a bus powered portable 2.5" inch drive. Although they may be a bit more expensive, I highly recommend going with a FireWire or Combo USB/FireWire drive over a USB only drive as FireWire performance (especially on the Mac) still trumps USB 2.0. There are a variety of good drives out there but to recommend just one I'll go with the G-Drive mini from G-Technology (from $159 for 120GB).
Just to round things out I'll suggest a few less expensive ideas as well. ZIP-LINQ cables ($5-$10 each) are a great way to keep cable clutter to a minimum. These cables retract on center spools and are available a wide variety of types (USB, FireWire, ethernet, etc.). I keep a handful in a cable bag. Flash drives have gotten so inexpensive that they can easily be had for less than $20, each with substantial capacities (2Gb). Various vendors offer trackpad and wrist rest area protectors, also for less than $20.