What its for

This guide allows you to application unlock your device, allowing it to run applications that otherwise may not run due to extra secutiry measures added to the device by some carriers.

Is it safe?
The procedure in its own right is safe, you wont break your phone by following the steps to unlock.

You should be aware that you are removing security features of the device, so after unlocking your phone may not stop an application from running which can have a bad effect. However there is no gaurentee that a similar program could not have a similar effect and run on a locked phone.

I think the best defence against badly coded programs is a forum like MoDaCo. If you are not sure of the source of a file you can ask, hopefully another memeber will have delt with It before and can give reassurance regarding the safety of the file.
Does it have a lasting effect?

No - you can relock your device via the unlock site. Hard resetting your device also relocks the device.
How to:

After reading the above click the link below to visit the unlock site. On that site you may notice there is no mention of the c500, however this doesnt stop it working.

http://developer.orangews.com/orgspv/comdefq.aspx

Select "Disable Certificate Security"

Then click proceed

Next read and agree to the terms and conditions

Enter phone details as required then click OK

At this point you have to connect your phone to the internet, If you are in the uk you can use pass through, outside uk then you must connect via pass through (put phone in cradle and allow the phone to connect using the pc's connection.

After connecting to the net you should get this message on the phone confirming unlock
Thats all. your phone is now unlocked!

Discussion, help and advice in the thread HERE