What is and isn't cheating

Now on wiki!

What IS considered cheating:

No-Fail (legal if you want to prove cutoffs)
Easy-Expert
Auto-kick
Stage Mode
A Bot
Using tape/rubber bands to hold down a fret
Having someone else hold down a fret, strum, or hit select (guitar) or having someone else hit a pad or kick the bass pedal (drums)
Hacking of any kind (i.e. custom songs; an exact replica of a chart in Game_GuitarHero GH1∞ put into Game_GuitarHeroIIILegendsOfRock GH3∞ cannot be used for scoring)
According to Deimos∞, Precision Mode is a cheat
Pro-Face-Off Pro Face Off Mode∞. Scores that you get from PFO are considered illegitimate because you don't have to save SP for parts you would have failed otherwise. You must prove you can pass the whole song without SP before you can submit a PFO score.
Glitches (i.e. the practice mode glitch in GH2.). Some glitches may be legal you should probably check with someone here though
Submitting scores across systems (i.e. submitting a score you earned on 360 under Wii)
Submitting a rounded or false score. Even rounding down is considered an illegitimate score.
Submitting single-player scores when played with more than one instrument. See this thread for full explanation.

What is NOT considered cheating:

Using a standard controller
Hyperspeed / Breakneck Speed
Large Gems
Any thing that alters the appearance of your guitar or guitarist (i.e. air guitar)
Select Venue / New Venues Only
Awesomeness Detection
Using a rubber band to fix your whammy bar
Using any body part outside of the ordinary. You could strum with your dick for all i care, its legal
Using Select to activate star power.
Performance Mode
Using a non automated object to strum (i.e. a pick).
Looking in a mirror for lefty flip (in league play this is considered cheating)
A Hyperspeed towel
Wearing gloves
Using baby powder or other things to keep your hands dry
Using grease or other things to keep your hands wet
Squeezing
Taping rubber to the strum bar (in essence this is basically wearing gloves)
Playing keyboard style
Having someone make a path for you
Playing a non-modified song on a custom game (i.e., playing Song_GH2_FreeBird Free Bird∞ on a custom Game_GuitarHeroII GH2∞ game)
Double bass (be it a custom or a store-bought mod)

What is not considered cheating, but is frowned upon:

Switching controllers in the middle of a song (according to kuh-ray-zee this is just frowned upon. This is still under debate but for now...)
Adjusting calibration settings to alter optimal paths

What you should go to the Hardware∞ forums about:

Homemade mods to the guitar/drums/controller that involves taking apart the guitar, drums, or controller

What we aren't quite sure about yet:

Pausing in the middle of a song
Using a drum pad (see FlorisJan's post∞) to play guitar

Obviously this is oversimplified, but you if feel there is something else that needs to be added, feel free to drop me feedback.
The guide in its original form can be found here: http://www.scorehero.com/forum/viewtopic.php?t=72918∞