[spazmaster]

Also SMS fits 160 chars, not 140.

[barredo]

20 chars were reserved for twitter username

[blasdel]

160 7-bit characters.

Twitter usernames are 15, the ": " takes another two, and three are reserved for future sigil retardation.

[papachito]

140 doesn't include the user name, the app name and probably some more info.