As long as you don't put your ps3 is dev mode using the exploit and then go online you should be safe. At the moment we don't know anything about what sony knows or sees while online and in dev mode, only time will tell. For right now its safer to stay offline while in dev mode.
This could work, but as NinjaOptimus said, soon you will have to update to be able to play online and then you'll certainly get banned if you try to use the backup manager or PSJailbreak,or whatever you use. It's also possible that a log is kept on the PS3's internal memory (not the HDD). I know I would do it that way.
You will get banned even if you use the device once and go on to back up manager and then go online because it will be in the log that you have used back up manager (LAUN-12345) and your mac address will get banned.