yes, there are costs, but there are costs in NOT doing this, and although these are somewhat intangible they should not be underestimated: they include customer confidence & goodwill. Ask Nvidia why they continue to update drivers for legacy hardware!

Meanwhile there are likely to be tangible benefits: the chips in the N800 aren't unique as there must be variants and derivatives used elsewhere, so a high quality OSS driver will increase the value of the intellectual property embodied in the hardware designs. Ask Atheros and Ralink what they gained from the OSS community!

Anyway, this is good news. It increases the likelihood of me buying my next internet walk-about device from Nokia substantially!