I have to agree with the above-when the rubber meets the road it's really difficult to top a Pismo even if you can get faster(absolute) clock speeds in the white iBooks.
Like a lot of folks, I can take a Pismo apart in my sleep and something like a hard drive swap takes about 10 minutes. After 10 minutes of working on an iBook(whether a Clamshell, Snow, or G4) I'm still trying to figure out just where and how hard I have to pry to get a part loose but not break something.
Although I've run OS X on a bunch of different iBook G3s(and even have a 900mhz running the Leopard 2006 WWDC preview) I have to agree that PIsmos and king in OS X. Heck, with a G4 upgrade, you can even run a bone-stock Leopard install. It won't be the fastest in the world, but will run. 1gb of PC-100 RAM makes Tiger plenty peppy, especially if you throw in a fast hard drive or SSD. As mentioned, you can stick a Carbus Airport Extreme compatible card in one and get access to WPA2 networks in Tiger. Put in a USB 2.0 card if you need faster USB I/O. You can't beat the variety of stuff offered for the right hand bay. I know someone who put a Superdrive in the stock DVD carrier, something that I keep meaning to try.
With two good batteries, you can sometimes push a Pismo to 10 hours. Granted I know "good" is a key phrase, but none the less I've done it in OS 9. The new Macbook Pro can't do that
.
I have a G4-upgraded Lombard and it's a nice running machine, although it is handicapped in OS X with 512mb of RAM. I've also been trying to get Leopard running on it using an install that a friend put together(It boots perfectly on G4-upgraded Beige and B&W G3s) but am still having trouble getting it to work.
All of that aside, my "on the go" OS 9 laptop is generally a TiBook, and specifically a 1ghz one. Although it's longer than the Lombard/Pismo, it's smaller in every other dimension and also lighter. Although I don't boot mine into OS X that often, it does a good job of running Leopard-there again the 1gb of PC-133 helps a lot. Also, the same thing about Carbus WiFi cards applies. The later generation TiBooks have a Radeon 9000, which isn't exactly cutting edge even by PowerPC standards but really perks these computers up relative to the Rage graphics in the G3s and early Tis. Having Quartz Extreme really helps in OS X.
With all of that said, there's one application where I find the Clamshells to be by preferred laptop. Many old full screen games were designed to run at 640x480. LCDs often look like crap when they're run at something other than their native resolution. Thus, I find the 800x600 screen of the Clamshell a nice compromise for this since the "window" still fills most of the screen rather than being a tiny square like on a TiBook(or a G3 Powerbook to a lesser extent).