These are the official G50 values as per the ICC operating manual for Standard Twenty20 International Match Playing Conditions ammended on 4th July 2006
For matches involving ICC full member nations, including Under-19 international matches, or for matches between teams that play first class cricket, the value of G50 to be used at present is 235. For lower levels of the game, G50 should be chosen to represent the average score expected from the team batting first in an uninterrupted 50 overs-per-innings match. For teams between associate ICC member nations, the recommended value of G50 is 190; for women’s ODIs, the recommended value is 175. For U15 internationals, the recommended value is 200. For other levels of the game, advice should be sought from the ICC.
Duckworth-Lewis.com team comments:
Apologise to everyone but this official statement still leaves plenty of ambiguity for T20 matches and it does seem like this rule was copied and pasted from the ODI manual without much thought. The latest 2007 manual is even worse because it doesn't explain any revisions of DL.