Hello, I need to change my Mac Adress, i tried following this guide:

http://osxdaily.com/2008/01/17/how-to-spoof-your-mac-address-in-mac-os-x/

I changed it, but after doing that, the connection doesn't work anymore and I've to set up the old mac adress to let it work again... I think there's a different process in Lion to do that, someone can help me?

After changing your MAC Address - did you restart your modem, or other connection?
A cable modem, for example, will be tied to only one MAC address at a time. Restarting the cable modem will reset that 'tie', and the modem should then connect with the new MAC address.
You might also have an internet supplier who requires you to contact the supplier with your change of MAC address - but usually a restart of the modem is all that's required.

If you are NOT connecting directly to the router, then also try restarting your router, which is the device actually used to share your internet connection.

I tried restarting the router but it doesn't solve the problem... Airport shows the message "Can't find an IP adress" and then "IP auto-assigned"... But the connection doesn't work and on airport icon there's an exclamation mark...

Test again with a direct connect to the router - plug in an ethernet patch cable from your Mac to the router. Does it find a proper IP address then? A proper IP address would be something in the range that your router supplies - perhaps 192.x.x.x, and NOT one in the range of 169.x.x.x

With ethernet it works, but I need to do that with airport...

Of course, the ethernet port and your wireless card each have different MAC addresses, so it's not a real test - sorry for wasting your time with that step.

Maybe you are changing the Airport card to a MAC address that is simply not valid?
Try turning OFF the wireless security in the router - just to check if that allows the wireless connection.
Did you restart your Mac after changing the airport card MAC address?
Does your wireless router allow MAC address spoofing?

And, the last question - is all of this worth the effort? I typically see this question from users who have been banned from another forum, or a gaming site.

Is that your position?
- Dale

No, I've not been banned, I'm just testing the security of my connection, but If I can't change the mac adress I can't do that, anyway, If I restart the computer, the mac address change into the original, every security protection is disabled, and router settings allow every mac adresses.. Moreover If I change the mac adress I can't connect to any connection, than It can't be a router problem... The router I use is D-LINK 2740b...

upon rebooting all spoofed MAC addresses will be returned to the original values as set in the hardware at the factory
there's no such thing as an invalid MAC address as long as you remember that the values have to be hexadecimal numbers.
the MAC address is the hardware address. A router assigns an IP-address to a MAC address.

As far as I remember in my experience, you'll have to turn the interface off, then change the MAC address, and then turn the interface on again

Turning the interface on and off has to be done both in Terminal as well as in the Network pref.pane
In Terminal you'll have to issue this command :
ifconfig en1 down and then ifconfig en1 up

So the complete procedure is as follows:

turn WiFi off in Terminal
change the MAC address in Terminal
turn WiFi on in Terminal
turn Wifi off in Network pref.pane
turn Wifi on in Network pref.pane
ask for a new DHCP release in Network pref.pane

Please note, that both Network Utility as well as the Network pref.pane will still report the original factory hardware MAC address

If you are successful your router ought to report the new MAC address as having been assigned an IP-number. The original MAC-address is possibly also still there as the lease might not have expired yet.