I like that alot alot alot nsruf!!! Here's my own revised version with your idea incorporated.
1. Roll 3d6 for each stat in order and take all scores where they fall - no rearranging or reassigning.
2. At the DM's option you may be allowed to swap a single pair of ability scores.*
3. Calculate your point total as if you had created this character with the standard point buy rules.
4. Consult your DM regarding the baseline for the campaign (25 point, 28 point, 32 point, etc.).**
5. If your character as rolled has a total point value greater than the baseline amount, congratulations! You get 4 additional points to spend at will as per the point buy rules.
6. If your character as rolled has a point total less than the baseline amount, subtract your total from the baseline. You may spend the greater of 4 or this value at will as per the point buy rules.
* I added this step because if step 5 applies to you, you would otherwise have very limited control over your character's stats, thus limited opportunity to play the class you want effectively.
** When determining this baseline, DM's should consider that it is possible for a character to end up with a character 4 or more points higher than this value. This is not a hard upper limit as it is under standard point buy rules. It is, rather, a minimum.
Now there's is still one variables that need to be tweaked to get the system working - the minimum of 4 points to spend. I'm still not sure about this value. What do you think now?