OK. I figured it out. The problem was a loose keyboard cable. To get to the cable, you've got to pull out the keyboard. You need a small phillips screwdriver.
1. Shut the computer down - remove battery
2. Unscrew the four screws of the rectangular cover just above the battery release. This exposes the memory expansion slot. For safety, remove the memory chip. At the very edge of the exposed area is a black screw holding down what looks like a clip. Loosen the screw. This screw goes into the back of the keyboard.
3. Pop the F1, F2, F11, and F12 keys. It may feel like you're about to break the keys, but they do come off and snap back on. Remove the small tabs if necessary to expose the screws that are between the keys. Remove both screws.
4. Lift the keyboard up from the top and pull gently toward the screen until the tabs holding the keyboard at the bottom slide out. Be careful not to pull up too hard, so as not to damage the cable underneath.
5. The cable is held to the board inside the computer by silver foil tape. Since the person who replaced your drive already had to remove the keyboard once, the tape is probably royally crinkled, if it is there at all. You may need to gently pull the tape up to straighten it out, or pull it off entirely. If you have to remove the tape, replace it with something appropriate. The cable may not stay in without the tape.
6. Gently push the cable connector back into the slot on the board until it is snugly in place. Replace tape. The tape will form a depression. That is, the tape will not look level-smooth. It will dip from the case down to the connector.
7. Reverse the procedure to replace the keyboard.

Suddenly my Powerbook G4 12" is having the same problem you are describing - space bar, delete key, tab, quotation mark... Anyway, did you ever find out what was wrong? Does anyone else know?

I'm having some real concerns about a repair just done on my PowerBook G4, 12''. I had destroyed the CD-RW/DVD drive someone - it was making awful noises and the motor finally just died - and went to a local guy in DC to buy the part and do the install.  

He was doing the work while was out of town, and it took him a lot longer than he'd promised when I showed him the machine. He sent some worried-sounding emails that he was going to ask a friend with engineering skills to assist, because the small frame of the 12'' made it really hard to do the installation.  

Well, he got the machine back yesterday. When he brought it back it was not booting up successfully at the start, but did after a few tries. It read a DVD-ROM and later imported a CD into iTunes successfully. However, the delete key, space bar, tab key and a few others are not working now, and there's a gap of 1-2 mm between the keyboard and aluminum frame of the laptop on the right side (i.e., above the optical drive). If I pound on the delete key and space bar they sometimes work, but it's clearly not right. Also, the clasp that opens the laptop is not releasing without a great deal of effort.  

Based on the evidence, it's pretty clear to me that the optical drive was not installed properly -- if it were, the 1-2 mm gap wouldn't be there. The service provider is very reticent to admit that his repairs caused the problems, but it's pretty clear that the keyboard and clasp worked fine before he and his engineer buddy opened up the machine. He's agreed to take it back on Wednesday and look at it and try to resolve the problem, but in the meantime -- is there any other possible explanation or fix for the keyboard issue?  

In the meantime, folks in the DC area, consider this a warning for anyone about Chris Barylick at www.macdweeb.com/.