For those Macs that can boot into the 64-bit kernel, there are several ways to force that to happen. Here are some pointers, and a gotcha, on how to make it permanent.
Note, this HOWTO is intended for reasonably savvy Mac users who want to learn more about their Mac and take advantage of 64-bit mode. If you have questions about anything, post them in the comments, and we'll all try to help. If anything here makes you feel that you're over your head, just pass.
First, if you need to check whether your Mac can boot into a 64-bit kernel, here's a reference to a TMO article that explains and provides list the current Macs that can.
Next, there are several ways to force a boot into the 64-bit kernel.
1. Hold down the "6" and the "4" keys at boot. This method only applies to the current boot and is not permanent. This is a good technique to try when you first embark on the 64-bit road to make sure everything works as expected. If so, you may want to make it permanent. To determine if the boot was successful, look at the System Profiler -> Software and the next to last line. It should say: "64-bit Kernel and Extensions: Yes"
Not only can this app set the boot mode, but will also provide additional information about your system. However, there is one caveat: This app converts the applicable plist file from human readable text file to a binary. That's fine. The system can still read the file, but I cannot with a simple editor, and that annoyed me. So I prefer the third method: keep the plist file as a text file and edit it directly.
3. Edit com.apple.Boot.plist. This file is found in /Library/Preferences/SystemConfiguration. Here's how to do it safely.
1. Make a copy of the file, in the terminal, for safekeeping:
$ sudo cp com.apple.Boot.plist com.apple.Boot.copy.plist
The file looks like this:
2. In the line after "Kernel Flags," line 9, edit it to look like this instead.
That file is owned by root, so you'll have to be logged on as an administrator of the Mac and authenticate with your password when you edit the file with, say, BBEdit or TextEdit.
When using BBEdit (or any other GUI text editor) make sure the default for an edited text file has the right kind of line break. It should be UNIX (Line Feed). Set or check this in BBEdit -> Preferences -> Text Files. Apple's TextEdit uses the UNIX convention, so you're okay. (Not a word from the UNIX gurus about "vi," you hear?)
From now on, when you boot your 64-bit kernel capable Mac, it will boot into 64-bit mode.