Hello,
Is it true that if I restore a jailbroken iPhone I will need to unlock it again?
I need to restore because I am selling my phone.
Is it possible to restore it without having to send it out to someone to get it unlocked again?

Thanks!
First things first, there is a difference between jailbreaking and unlocking.
Jailbreaking the device allows you to write unsigned binaries (i.e., applications not signed by Apple).
Unlocking allows you to use SIM cards from other operators.
In order to unlock the device, you need to jailbreak it first.
When a device is restored, you lose your jailbreak and your unlock.
The tricky part is, the unlock is baseband dependent, not firmware dependent.
Never restore using iTunes if you want to keep the unlock, instead, you need to download the correct IPA file (the newest firmware) and use something like PwnageTool to modify the firmware and make sure you baseband is not upgraded. You then restore the modified IPSW file and download UtraSnow from Cydia to unlock.
I am not going into details here since this is covered extensively all around the web.

I hope this helps.

Edit: IPSW not IPA, thanks EddieEC.
Jadz, before you restore it, you could take it to some shop, and get a Gevey sim card which will unlock it without a jailbreak, but it's BaseBand dependent so make sure you preserve the baseband.
the thing is i heard that you can perform a restore without installing or downloading anything, maybe its something from inside the iphone?