I know there is really no difference, but is 'LEFT JOIN' an ANSI form or are there any RDBMS's that will fail 'LEFT JOIN' and require 'LEFT OUTER JOIN'. [I am asking here so I can save a few clicks, form fillings, etc to get the correct ANSI standard!]
[OUTER] is optional, per the ANSI spec (92, but I'm sure later versions also cover it).
Of course, you're assuming that every SQL product is ANSI compatible. For joins, they probably are..