I don't have any speech problems yet, but , being a computer geek by trade, I've studied the AAC options for PALS for the last year and a half. The best option that I see these days is to buy one of the smaller netbook computers running one of the Windows operating systems. The reason I recommend this is flexibility.
1) There is a ton of free accessibility software out there for Windows machines. Programs to help with typing. Programs that replace the mouse buttons. Programs that allow you to use a webcam to control your mouse pointer. Programs that will allow you to type a certain key sequence and see a whole sentence appear onscreen. And many others. All free.
2) When your PALS becomes more disabled, there are all kinds of commercial AAC software offerings out there, and the overwhelming majority of those commercial programs run on Windows. Things like eyegaze control of the computer. Natural sounding synthetic voices.
3) Comparatively speaking, netbooks are dirt-cheap. I've seen some good ones on sale for as little as $300 US.
4) If your PALS is already comfortable with Windows PCs, there is nothing to learn but how to use the new AAC software. If you use free AAC software , you can install your PALS preferred software on every computer in your house, which provides instant redundancy. If your PALS' primary communication device fails, then you have backup devices for your PALS to use until the primary device is repaired or replaced.
5) Bigger devices are better. iPhones, iPods, etc. are small and cute and portable, but they only work for your PALS as long as the PALS has reasonably normal use of his/her hands. If you want to know how it's going to work when your PALS's hands start going, put on the thickest pair of gloves you own and see how it works. After you struggled to type a single sentence for about ten minutes and you want to rip off the gloves and throw them across the room, stop and think "The spot I have picked for my gloves to land-- that would look like what with an iPod/iPhone/Palmtop sitting in the middle of it -- possibly in a couple of pieces?" This PALS may not be able to operate all the fiddly little buttons on one of those cute little gadgets, but I can still fling them across all the rooms in my house.
Just a few thoughts from one of your gadget-friendly PALS.