There was no real need to patch the CommCenter executable.
The CommCenter patches were initially released as plain patchfiles that required entering shell commands, but later appeared in Cydia repositories too.
With 4.0.1, it is required to do it this way. So you don't need an unlock hack installed per say. The CommCenter patch can be found in some shady repository you might need to add. (Try searching on myrepospace.com)
The CommCenter patch makes it accept any non-matching signature in the carrier bundle.
Anyway, it is not that difficult and can be read about on other sites. I got all the handy tools on my iPhone already (ssh with changed passwords, vim), so here's what I did... (you should use FTP if you don't want to mess with SSH)
cd /var/mobile/Library/Carrier\ Bundle.bundle
plutil -convert xml1 carrier.plist
Google for typemask.png to see what bits to toggle.
Don't forget to make a backup of the file if you are unsure.
Then, for some reason you need to reset the network settings. Perhaps it just updates /var/mobile/Library/Preferences/com.apple.carrier.plist I don't know exactly yet)
Perhaps you could try
killall CommCenter SpringBoardtoo.
Btw, The directory is actually a symbolic link and the files in it gets updated occasionally by Apple. It will ask you however before doing that.
To recreate, use something like:
ln -s /System/Library/Carrier Bundles/TMobile_nl.bundle /var/mobile/Library/Carrier\ Bundle.bundle
While MyWi (in Cydia) offers internet tethering too, it got it's limitations (but is being worked on). e.g. it cannot connect via bluetooth to a laptop. Also, it costs a bit of money.
NOTE: Your mobile provider probably does not accept free internet tethering.