Don't you just love the advent of number portability? It makes it so much more convenient to hop from one mobile phone provider to another! Combine that with Amazon's great mobile phone offers and a one-year contract, and you've got the makings for some pretty slick deals.
Case in point...every year for the past 4 years, I've received a brand new mobile phone absolutely free. In fact, I've often even been "paid" to pickup this brand new phone. You see, the mobile phone market is so competitive that the players in the market are willing to heavily subsidize phone costs in exchange for a one year (or more) contract. If you are willing to switch providers each year, you are basically guaranteed a free (often quite nice) phone every year, and you can still keep the same phone number.
I've just gone through my annual mobile phone procurement, and made out quite well, if I must say myself. Last year, I got an amazing deal through Amazon, and was able to pickup an Audiovox SMT-5600 for free after rebate when signing up for a year contract with AT&T Wireless (now Cingular.) Now, this phone absolutely ROCKS. It's a Windows Mobile 2003 smart phone, which means, it basically does everything my old Pocket PC did, including Outlook synchronization, mp3/wma playback, MSN Messenger, etc. Granted, I don't have a data plan, but still, this is a great phone, all in a compact form factor and a large screen.
Of course, when it came to Cingular's GSM network capabilities, I wasn't too impressed. Service was decent, but from my condo was often pitiful. I longed for the days of my prior T-Mobile service. No worries...my contract with Cingular would be up soon enough. It ended today 
Needless to say, I planned on keeping my 5600 phone, but there was one problem. It was locked for use with AT&T/Cingular service only. Never to fear...the "internets" are here! A simple Google search lead me to this rockin' site. It worked BEAUTIFULLY! In a matter of minutes, I had unlocked my phone for free! Another site allowed me to get rid of all the nasty AT&T branding, setting me up for a seamless transfer over to T-Mobile.
The last step was finding the best deal on a new T-Mobile phone. Again, I went to Amazon and found this excellent offer, which will PAY ME $50 (after rebates) to pickup a Motorola RAZR V3 phone with a year contract. I saw that these phones were going for over $200 on EBay, so I knew this was the offer to grab. Got the phone today, popped my new T-Mobile SIM into the Audiovox, and I'm back with T-Mobile superior network. Filled out the rebate cards, mailed them, and put my RAZR up for auction on Ebay. In the end, I figure I'll make around $250 or more just for switching carriers. Rock on!!
[UPDATE:]
The phone went for $186 and $12 shipping. Not bad, eh?