My Bad...
It is not a true ground but what will happen is you dissapate the static to the floor as long as you are holding the case if you let go of the case and then touch it again you may shock the system... the best thing to do would be to go buy the proper ground work mat and static wrist gaurd... or I know people that have taken old power bars and cliped the positive and negative leads off leaving the ground pin then you pulg into this and the wall and tape the bar to the service outlet, I would not do this it may come back to hunt you and I am would not tell anyone ot modify something to do that kind of job.
keeping a good humidity level having all your stuff right there and even more so having ground mats is the best but if you dont have them keep you feet planted, not on carpet. I was once told in another forum that if you have something els that runs on a 3 prong, take a jumper and connect the two frames to ground out the system you are working on, this tech allso said the other computer or what ever should be OFF when doing this... I have not realy looked into this but it may be a simple and safer way to "Jerry-rig" a ground as you just need wire that you can fix to the two systems fermly with gator clips or tape and this would be a passive ground as the other computer frame is only conected to the ground.
As far as working with an ATX system you always have power to the Mainboard if it is pluged into a service outlet and there are people killed every year working this way and hundreds of Mainboards and other parts cooked because people work like this. You want to have a passive ground... I would use the gound jerry rig idea before having a system plugged in when working on it as the most reasent A+ books are telling you not to ever work on a system plugged as there is still power sent to the mainboards.
If you think I am pulling your leg leave your system plugged in turn it off, open it up, find where your power button leads to, it will be a point on the Motherboard, pull the leads off the motherboard and use a #1 or 2 flathead and jump the two pins... now tell me what is happening on you monitor. your sytem is booting the power controls are not solid by any safety standard, aside form India... (Indian safety standards are low if there are any at all) As long as there is power going to you r PSU you have power os the Motherboard.
Either way it is your life and your computer and nomater what you do remeber this be carefull and look at what you are making contact with, every part of you your tools and the computer parts you are working on and a around... That and unplug SATA drives any time you work around them they brake easy and last thing you need to be doing is running around the city looking for Lapage model glue and then very carfully placing the lead plate back on flush with the pins without getting the glue on the pins, I did that once and never again... It is off topic but a good thing to know.