First, the 770 was not an N-series device. There's no unambiguous progression from 7710 -> 770 -> N800/N810 -> ??? to know what the next device is, so I'd consider any particular guess likelier to be wrong than right. (IIRC, the most popular guess for the N800's name was 880, and the N caught everyone by surprise. N900 was floated as a name for the N810.)

I don't think Nokia is likely to let the model number slip before an actual announcement, but thanks to the pre-alpha and alpha SDK releases, we know Fremantle initially contained support for a hardware platform named RX-51 and support for an RX-71 is being added. (For comparison, N800 is RX-34, N810 is RX-44, and N810W is RX-48.) It's logical to assume the RX-51 is the lead device, although there's a very slight possibility it actually is some Nokia internal testing mockup.