Alesis Andromeda A6

I think the Ion is a good synth to start with, but I started with 
the Korg MS2000. It was a great synth to start with as it wasn't too 
expensive, and you can start programming with a simple sin wave and 
begin to add complexity to the patch to see what each new addition 
does. I will always have a place in my heart for this machine.

If you can afford it, the Nord Modular G2 is also a good machine to 
start learning with. You can actually choose the modules to add to 
the patch, see the results on both the synth and your computer, and 
hear the results through the synth. I have fallen in love with mine. 
Probably the most versatile synth I own, next to my real analogue 
modular.

Going cheaply, you might try the Arturia MoogV software. It's a 
software emulator of a Moog, and so you can play with patching the 
modules together as if you had a real modular in your livingroom. 
Great for learning about patching any kind of modular synth, and a 
cheap way to learn compared to a hardware synth. The sound is close 
to the real thing, but nothing compares to hearing a real analogue 
modular.

Joe