A Wiki listing the current alternative activation methods:
The content of the link above. Snapshot 7/5/2007. I claim no credit, obviously...
Activating an iPhone via AT&T Loophole
Originally posted at InformIT.com by Seth Fogie
1.) Be an AT&T customer
2.) 'Upgrade' your phone/plan to the iPhone and get activated and connected.
3.) Return your iPhone and switch back to original plan on original phone (free for at least 14 days).
4.) Tell them you want to keep the iPhone for and assign it to a new account that is separate from your original.
5.) Take iPhone home and remove SIM in top of device.
6.) Never activate iPhone and just use it as iPDA with full Wi-Fi and iPod support.
Activating an iPhone using another iPhone
Originally posted at The iPhone Blog
And stolen without credit by Gizmodo
1.) Obtain 2 iPhones
2.) Plug in iPhone #1 In iTunes select "I am a new AT&T Wireless Customer" and "Activate 2 or more phones on an individual or FamilyTalk Plan."
3.) Follow the steps for the FamilyTalk plan and enter "Cell Number X" to port a number over from another provider (e.g. Sprint)
4.) When the you receive the "Activation Complete" e-mail, plug in iPhone #2.
5.) Select the option "I am an existing AT&T (Cingular) wireless customer" and "Replace a phone on my account with this iPhone"
6.) Fill in the information re-using "Cell Number X." Allow the iPhone #2 to activate using this number. This number will be legit.
7.) Plug in iPhone #2, it will unlock the phone for use, but without a cell phone number assigned or account from AT&T.
 Activation with a Pre-paid-Plan
Originally posted at TUAW
1.) Buy the iPhone.
2.) Connect it to iTunes.
3.) Sign up using 999-99-9999 as your social security number.
4.) After failing the credit check, select a GoPhone plan.
5.) After signing up for a GoPhone plan and being assigned a number and passcode, connect to the AT&T GoPhone funding page as prompted, enter your credit card or debit card information and you're good to go. DO NOT attempt to fund your iPhone over the phone with AT&T. DO NOT set up your iPhone prepaid account in advance with AT&T.
 Activation via Jon Lech Johansen’s PAS (Phone Activation Server)
This method works by spoofing Apple's activation server. Details here. Instructions step by step:
1). Download UltraEdit-32, install it.
2). Download PhoneActivationServerV1.0, extract it to desktop or anywhere you want.
3). You might wanna backup the original iTunes.exe(located under C:\Program Files\iTunes) first.
4). Run UltraEdit-32, open file iTunes.exe, use Ctrl+G to go to address 2048912, then enter 33C0C3. Do the same for the rest two offsets: Go to address 257074 then enter 28 and go to 257013 then enter 33C9B1. Save the file and close UltraEdit-32.
5). Open Windows explorer and go to c:\windows\system32\drivers\etc. Open the "hosts" file in ULTRAEDIT and add the line
to it. This will redirect any DNS query of "albert.apple.com" to your local host. Save & exit.
6). All you need to do is to run Phone Activation Server V1.0 first, leave it running and then run iTunes. Now when you plug in your iphone it will activate automatically in about 60 seconds.
Note though - This does not unlock your phone so you can use it on other networks. It only activates the phone so you can use it as a PDA-ish device. Surf the web using WiFi and such. Although I'm not entirely sure about the GoPhone system - I'm under the impression that it activates the phone, and you'll have a number with minutes? ::shrugs:: Not enough spare change to test it out, unfortunately.
 Heh! They're afraid of being Dugg/Slashdotted.