Originally Palm had two functions planned for the side button. In reality, only the Hold Side Button function is configurable in the Settings | Personal | Buttons page.

Using a registry editor you can easily fix this. Go to HKLM\Software\Microsoft\Shell\Keys

Create a new key called "40C7" and copy all entries from the "40C6" key, or manually recreate them. In the example below, I'll configure the side button to launch Voice Command in PTT mode (push to talk).

HKLM\Software\Microsoft\Shell\Keys\40C7:
BtnWnd=0
Default="\windows\voicecmd.exe" -ptt
Flags=0
Icon=\windows\voicecmd.exe,0
Name=Push Side
ResetCmd="\Windows\AppButtons\Camera.lnk"
WndCls=

If all goes well you can open Settings|Personal|Buttons and you'll see the new Push Side function available, already linked to the VoiceCmd applet.