Questions you've asked (and questions I hope you ask)
A: SleepTight is generally compatible with any Macintosh model or screensaver. SleepTight has been extensively tested on G4 desktop Macs as well as PowerBook models including the PowerBook G3 (Pismo), Titanium PowerBook G4 (DVI) and Aluminum PowerBook G4 (12 inch).
However, some users report issues with certain combinations of Macintosh model and screensaver plugin. These issues are not directly caused by SleepTight, but instead appear to be caused by an interaction between some screensavers and specific video hardware/drivers.
Some screensaver plugins, particularly third party OpenGL screensaver plugins, will crash when run right before sleep. These modules seem to have particular issues with older PowerBooks, especially PowerBook G3 (Wallstreet) and similar models with older 3D acceleration hardware. Note, however, that potentially these crashes could affect any Macintosh.
Unfortunately it is not possible for me to test every possible combination of screensaver and Macintosh. I can only recommend that you use the Apple-supplied screensavers or test your screensaver with SleepTight before using it permanently. While testing you should be sure to save your work before putting your machine to sleep.
If despite your testing and precautions, you do suffer a screensaver crash your machine may not awaken (will be in a "coma"). If this occurs you can usually use the Mac's power button to turn the machine off. Once off most desktop machines can simply be restarted normally using the power button without ill effect.
PowerBooks which suffer from a sleep "coma" may require a pushing the power button for longer periods (10 seconds or more) before they will power back on after a coma. PowerBook users may also need to reset the power manager after a sleep coma. The process to reset the power manager varies depending on PowerBook model, please see this article for more information.
A: SleepTight is only as secure as OS X's built in screensaver. Unfortunately this is not very secure. As of Mac OS X 10.2 (Jaguar) there are several well-known issues with the screensaver that may allow an intruder to bypass the screensaver lock.
First, some systemwide hotkeys will pass straight through the screensaver even when locked. In particular the "Full Keyboard Access" option of Mac OS X can be used to manipulate menus and windows even with the screensaver locked. The only workaround for this is to disable "Full Keyboard Access" in the Keyboard pane of System Preferences.
Applications which use global hotkeys are also a problem, as they may be activated even when the screensaver is locked. Of these perhaps the most dangerous are applications designed to kill processes (like Ambrosia Software's EscapePod) which can be used to kill the screensaver even when it is locked. At present the only workaround is to disable these utilities.
Next, there was a widely publicized method to crash the screensaver, involving typing exceptionally long strings into the password dialog box. A screensaver crash would return you to the desktop with full access. This bug was been corrected in Mac OS X Security Update 2003-07-14, machines without this update are vulnerable.
Finally, anyone with physical access to your machine can reboot the machine from the keyboard even with the screensaver locked. While there is little you can do to prevent this (no machine is secure if the intruder has physical access) you can take reasonable steps to protect yourself by password protecting your machine's boot sequence using Apple's Open Firmware Password utility.
A: SleepTight can use virtually any alphanumeric character or the F1-F15 keys for its lock hotkey. Hotkeys must have at least two modifier keys (Command, Shift, Option, Control) or contain one or more modifiers with an F-Key. The use of two modifier keys or F-Key helps prevent conflicts between SleepTight and the standard menu and system keys.
A: SleepTight registers its hotkey by keycode, the OS-level representation of the button on your keyboard. Unfortunately there is no easy way to determine the letter represented by the keycode for every possible language/keyboard combination. SleepTight uses its best guess, but it may be wrong. Luckily even if it gets the character wrong, the hotkey will still work.
The character to keycode map can be localized. If you are interested in helping localize SleepTight's keyboard map please contact me at the contact address on the About page
A: Effectively none. SleepTight is entirely event driven. Unless you hit the hotkey or put your system to sleep SleepTight uses no processor time.