Go to Settings>General>About. Look at your "Serial Number". Ofthe 4th and 5th digits (last 2 #s before the letters start) are LOWER than 40, you can have a standard jailbreak. If they're GREATER than 40, you'll have a tethered jailbreak.
October 09 is when it became a standard. If you got your phone before than you should be allright. If your wondering on if you will have a tethered jailbreak or not. I know I got my phone in September and I'm in the NON-tethered jailbreak class.
Yes it does. The rule of thumb is that whenever Apple releases a new firmware, DO NOT UPDATE. Go to iTunes, and JUST SAY NO when it asks you to update your OS/Firmware. Wait the 2-3 weeks for the jailbreak and unlock to be released. THEN update, jailbreak and unlock your iPhone. It's a pain in the ***, but it's well worth it to be able to keep the privelage of jailbreaking and unlocking.