I have the same boot loop on my 3GS. Searched on the net for solutions, nothing works. More than five hours work rebooting attempts with and without iTunes. Non of the web hints worked properly. First symptom was the touch screen not responding at all, none of the buttons, not the sweep to start and put the password. Then Aborted the process by pressing on/of button together with the home button, and restored via iTunes. then problems with the SIM code (please do unlock the SIM card on the phone -that isn't bloody working!!!). Then suddenly I got one step further, for some reason I could unlock the SIM card and the "restore from iTunes started; during this restoring proces, EVERYTHING worked, I could get in the phone and all buttons (as far as iTunes already restored them) worked properly...until the restoring process wash finished...the screen was blocked again and I landed in he same swamp..
Apple this is a disgrace !
By the way I replaced the battery with an iFixit kit a few months ago.... :-)