Once you empty the trash on your Macintosh, for all intents and purposes the data that was in the trash is gone. All of the space that was occupied by that data is now treated by the directory as empty, and new data will immediately be able to use that space.
It's true that the data that you trashed is not necessarily overwritten immediately. However, that is irrelevant with respect to the amount of free space left on your drive.
The only reason that some folks might care that the data isn't overwritten immediately is if they are concerned about someone gaining access to their computer and somehow retrieving that data. (e.g. if you have valuable trade secrets in the deleted files.) If this is what you are concerned about, then instead of simply trashing your data, you should choose Finder --> Secure Empty Trash when trashing your data. This will make sure that all of your data is overwritten immediately.
See:
http://docs.info.apple.com/article.html?path=Mac/10.5/en/8279.html
You can make this process easier with:
iShred
http://home.comcast.net/~jeff.ulicny/software/utils.html
As far as overwriting all of the files that you have *already* trashed, you could do this by backing up your drive, reformatting (erasing) your drive and writing zeros to it using Disk Utility, and then restoring your data.
An easier way to do this would be to open Disk Utility, and select the PARTITION of the disk that you want to Zero-out free space on (The partition is the child option that has the same name as the disk on your desktop: most likely "Macintosh HD"). Click on the Erase tab, and near the lower left, there will be a button to "Erase Free Space..." which, when you click it, gives you the option to zero-out all data, run a 7-pass erase, or run a 35-pass erase.
