If you own a Rooted Galaxy S Mesmerize running 2.1
and have a Mac, it's actually pretty darned easy to modify the status bar if you're rooted. Just change the .png inside of the "framework-res.apk".
No .apk signing, custom roms, flashing, or anything.
Just a simple extraction, edit, re-archive and overwrite!
Use Root Explorer (available on the market) to navigate to "/system/framework" on your phone and copy "framework-res.apk" to your sdcard. Then on your computer, extract it (I use stuff-it expander on OS X) navigate to "/res/drawable-hdpi" inside the folder and open "statusbar_background.9.png" in Photoshop.
Make your changes (I recommend small changes like making a grey bar, black, or changing the transparency) and then save it back with the exact same file name.
Zip the entire extracted folder (not just the files that were in the folder, but the ENTIRE folder that was extracted, and then option-click and select "Get Info" and in the info pane change the extension to .apk instead of .zip. OS X will ask you to confirm the change to .apk. Choose .apk. Copy your new "framework-res.apk" to your sdcard.
Then, on your phone, move the new "framework-res.apk" to the place you copied the original from and overwrite the old one (I recommend keeping a back-up of the original) and then reboot your phone. DO NOT RUN THE .APK OR INSTALL IT. COPY IT OVER THE ORIGINAL FILE IN ROOT EXPLORER.
Marvel at the wonder that is your new status-bar.
• I've only tested this on a Rooted Galaxy S Mesmerize running 2.1
• I am not responsible for anything that goes wrong.
I also wouldn't recommend using this method to change any other graphics in "/res/drawable-hdpi" because ".9.png"s have a special format that tells Android what parts of the image can be stretched. On the status bar it's not important since the entire thing is stretched, but funny things can happen with other graphics. I call this the easy(er) way because I'm in no way a programmer or Android guru, but still managed to pull it off. Post if you have questions, and I'll try my best to help.)
Here is my statusbar image.
And an example of what it looks like on my phone.
Hope this helps someone!