First of all... sorry for not being very active on the site lately.
On to the issue at hand... The HD started failing in my Tibook and since the case is destroyed and the battery is toast, I think it is time to turn it into a IMac style machine... So it will be getting a custom (hopefully laser-cut) case and I figure I'll have some extra space to add a reg size HD, and get more bang for my buck. 7200 rpm + 500GB. When I search for adapters, all I can find are ones to do the reverse. It is just hard to tell if they will work both ways. I assume that it is just a pin to pin connection, but some of the ebay items have resistors on them, which lead me to believe there may be some circuitry involved. Does anyone own one of these?
Does anyone foresee any other difficulties that may arise?
Thnks everone!
*note* On examining the pic a bit more I think the resistors are only to power the LED, but I think reversing the circuit could still f*ck it up. The solution may to just order one with no LED!

I own one. Not like the one picured, but it is a 2.5 to 3.5 adapter. No resistors, no chips, no LEDs. The pinout is identical with the last 2 pins for power. Wouldn't be hard to make one to fit your app.

It can be done but is a bit more involved than the simple adapter. Check out this guide:

http://www.applefritter.com/node/10859

I didn't think about the power supply. I forgot the 2.5 drives use 5v. This should work for power and you could make it very small. http://www.elecfree.com/electronic/lm2577-5v-to-12v-dc-converter-step-up-voltage-regulator/ The only problem is power output is only about 800ma. That may not be quite enough.

Great info!!! Thanks for the help!

You could always use a micro ATX or ITX power supply to power the system. I think a 100watt would be plenty to power everything.

.... That's exactly what I decided to do! I just bought a used pull on ebay for $4... 185w. I'm going to be adding a pcmcia card with usb2 and firewire, and it will also need 5v, so it looks like that should do the trick!
I also want to power the rest of the system with it. I can get the 24v needed for the powerbook from the two 12v from the mobo connector, but I must admit I am a bit sketched out to try it. The advantage would be reusing the power up button to boot up the system. (I suppose I could wire up my own relay, but simpler is better!)
I have also ordered the adapter, and a 2.5" to CF card adapter just for some experimenting... It might end up in my mini!
Thanks for the help y'all and if you have any more thoughts and suggestions, keep em coming!