Be careful guys... The guys who design the iriver proabably work under the assumption that the stock charger won't generate too much current. If you plug in a high powered charger with a stone dead battery, and they try to run the iriver, it might draw more current than some of the internal components will tolerate. I hate to see someone fry their recorder.
My boss just fried his cell phone the same way... and he's an electrical engineer. The polarity was right, he didn't wire it up wrong, but the big power supply he was using had more capacity than some component in the phone.
If you want a cheap alternative, buy or make an adapter that plugs into USB and into the charger port of the iRiver. It will charge a little slower, but it will max out at about 500ma and won't fry anything. I made mine. Teddy just showed me one he bought off ebay or somewhere for $1 plus a few dollars shipping.