I have never had to take a laptop with me on support calls. THe only time I've missed one was where the customers machine was so messed up it needed recovery, not support.
Here's a list of other things you should consider.
A Firewire drive with a bootable OS on it (you have to do a fair bit of work to make a universally bootable system) with enough room for data recovery
DiskWarrior, TechTool Pro and DataRescue all on CDs. Optionally, Drive Genius, other recovery tools. Retail version of Tiger.
CD(s) of the latest OS patches, application patches, good shareware downloads and Internet application installers like Firefox, etc. (don't take for granted your clients will have fast internet access. Installing the Software Updates on a new machine with a 'lite' broadband connection can take 2 hours. Dang near impossible on dialup. Much easier if you have a current set of downloads from Apple Support and can install locally)
The same again for OS9
TechTool Pro has a bootable Firewire 1 Gb Keychain drive with TechTool Pro on it, it's a couple of hundred dollars, but may be worth it.
Spare cables -- ethernet CAT5 a couple of each lengths incl. a 25 foot, USB, USB M-F Extender, Firewire, telephone, AC power.
PRAM batteries.
Spare RAM sticks in the most popular sizes.
Jewellers' screwdrivers. Torx drivers of all sizes including the 0 and 00 which are not commonly included in sets. (needed for Powerbook hard drive installations),
2 flexible putty knives for Mini opening,
Spare internet router/gateway/4 port hub,
2 or more USB 2.0 hubs, preferably with AC power,
couple of surge suppressor power bars, AC extension cord,
anti-static wrist strap,
canned air, contact cleaner,
iKlear or screen cleaner,
thermal grease (for iMac G4's),
blank CDs and DVDs.
RJ45 crimping tool and spare cable ends. Raw CAT5 cable. Cheat sheet to remind me the colour order for crimping raw CAT5 cable. Surface mount RJ45 jacks.
Tools (you can buy zippered pouches or briefcases with most of the common tools for computer repair, and then add your specific extras). Soldering iron, solder, electrical tape.
Spare jumpers for IDE hard drives.
DVI-VGA adaptor.
Microsoft Optical Wheel Mouse (wired).
Some of these things, of course, you will be selling to the customer... you carry them with you so that you can install them on the spot and not have to waste time on another trip.