Well one simple method is to wire a series diode or two between the battery positive to the Arduino...
Sounds interesting but TchnclFl reminded me that Ni-Mh batteries will drop tension very sooner before it being discharged.
I'm not sure how much you want to spend..
Those options sounds great CaptainObvious, thanks. Sad its the difference between the amount you want spend and the amount you can spend. I know you´re just being gently, understood, Im just kidding ;D From what I know will must import those with high taxes (incredible 80%), but it´s a good solution for some final setup but prohibitive for prototyping.
What about Lefty solution using Lithium ion batteries? Sounds reasonable? So maybe i dont get the exacts 5 volts all the time but it ends reliable ?
Cheers,
Rodrigo