Figured I would post this here in case someone hadn't heard about it. This USB Jig http://www.amazon.com/gp/product/B0053H73JQ/ref=ox_ya_os_product will automatically put your Samsung phone into download mode so that you can flash firmware with ODIN in case you accidentally flash a corrupted firmware or anything else that might brick your phone and not allow the button combo to put the phone into download mode. $7 is a small price to pay to avoid a useless brick. I have read claims that it works on both gingerbread and ICS and that it works with Epic 4G, Epic 4G Touch, Nexus S, Galaxy Nexus and a host of other Samsung phones.
This also resets the flash count, in case you need to return it for warranty issues you can reset the count and flash stock unrooted firmware and the phone will appear to have never been rooted. Samsung may or may not remove the ability for this to reset flash count, but it still can get into download mode.
There is a handy guide made on XDA for the international SGSII, but the process is pretty much the same for any phone. http://forum.xda-developers.com/showthread.php?t=1402286
I don't know if there is a similar jig for HTC phones, if anyone knows I would be interested to know. Motorola is said to have a "factory cord" or something that allows similar functionality on Motorola Phones, again, if anyone knows for sure, let me know.