I don't think that answer is entirely correct.
From: http://support.apple.com/kb/TS2756
"Note: If you leave the Personal Hotspot screen or put iPhone to sleep, iPhone only broadcasts the Wi-Fi network for an additional 90 seconds. If no devices join the Wi-Fi network during that time, iPhone 4 stops broadcasting your Wi-Fi network. You can start broadcasting your Wi-Fi network again by tapping Settings > Personal Hotspot."
So, if I understand correctly, if you leave your iphone on the the Settings --> Personal Hotspot screen, than it shouldn't stop broadcasting.