The Riebl VME bus ethernet board works great with NetBSD 5.1, with or without a battery installed. The battery just stores the hardware MAC address. The default MAC address will work fine unless another Riebl card exists on the same network and similarly uses the default MAC address.
You'll want to set the Riebl jumpers as described in the NetBSD Atari FAQ. In my case, I needed to purchase some extra jumpers at an electronics store. Then in order to get the card working I needed to pull all the original jumpers and re-seat them. After doing this my Riebl card started working properly and continues to work problem free.
You'll need an AUI transceiver to use the Riebl card on a 10baseT network. I use a Faralon Etherwave AUI transceiver. You should be able to find one of these fairly cheaply on eBay or elsewhere. You'll want to leave the SQE switch on the transceiver set to the ON position.
To set the MAC address, get the ANS (Atari Network Services) Software for the Riebl Ethernet card. Extract disk 1 to a 1.44MB floppy and boot into the BIOS version of TOS. (To do this, turn on the TT030 and as soon as the power light goes on, hold down the "Alternate" key. Once the memory tests pass and the hard drive countdown starts, release Alternate and press it a couple of times, then hold it down again. With Alternate held down, press Esc and wait a few seconds. You should see TOS appear.) You'll find an app called rcpcheck.tos on the root of the ANS floppy. Run it and you'll be prompted for the last two hex digits of the MAC address to set. The battery on the Riebl card obviously needs to be good for this to work. The main benefit you'll get out of all this is that you won't get a message about a missing battery on the Reibl card when booting NetBSD. Other than that, following this procedure isn't necessary to use a Riebl card with NetBSD.
If you can't get your hands on a Riebl card, try researching the BVME410 ethernet card, which appears to be more readily available. There appear to be some of these cards for sale here but it's unclear which one would be the best bet.