If you purchase it now I don't see a new
iOS on the way other than updates. In
that case, no, you shouldn't have to pay
for those updates.
However, when Apple goes to iOS5 I am
certain there will be a small upgrade fee
associated with that.
I am guessing all of this based on the fact
that I bought MyWi for $10 (now $20) under
iOS3. When iOS4 came out I paid a small
upgrade fee of $10. Not bad.
Usually companies charge you an upgrade
fee when the entire schematics of the OS
changes rather than just small updates.