On geohot's blog he says the following about the downgrade.

Here is a bspatch file to be applied to ICE2_02.28.00.fls allowing downgrades from 2.30.03 using BBUpdaterExtreme. By replacing the patched cert with your current run cert, you can downgrade from any other version.

Does anyone know how to downgrade the baseband from 4.24.02?

I have a 5.8 bootloader.

Thank for any advice.

I don't think anybody has been able to do so yet. There's some mentions in that posts's comments that no one's been able to confirm the run cert for the newer baseband/firmware (in order to patch the 2.28 files for the downgrade process). There's also mentions of unsuccessful attempts to downgrade from the 3.0 beta basebands in those comments.

I would like to try the 3.0 betas on my 3G (already have them on my backup 2G) and would like to downgrade if needed later. Maybe somebody will figure it out sometime before 3.0 releases publicly (either that or somehow manage to get the 2.28 baseband to operate with 3.0).

nobody wants 2.x anymore.

If anyone body is successful let me know too

fuzzyband downgrader in cydia now support 4.x basebands back to 2.2.1.

Obviously 5.8 bootloaders only

Used it on mine worked a treat.