Official updates - I suspect no drive will, though I may be mistaken. If there are any official upgrades, I expect that they'll be of the pay-for variety unless the alliance that controls the DVD-plus formats is not charging an extra royalty for adding DVD+R DL capability to an existing DVD+R(W) drive.
Unofficially, the ND-2510A (DVD+R DL capable version of the ND-2500A) might - the FCC filing for that drive was based on the hardware being the same as the ND-2100A (DVD-plus only version of the ND-2500A). However, whether drives sold as anything other than ND-2510A are upgradeable remains to be seen - they may, for example, lack the necessary factory calibration data for dual layer writing.
Dual layer DVD+R media is likely to be in very short supply for several months, and very expensive when it can be obtained. I suspect that it may be hard to obtain dual layer media to start with other than discs bundled with retail packed drives. A few dual layer coasters from a failed upgrade attempt may go some considerable way towards the cost of a new drive that's dual-layer capable from the factory.
As I've posted before in the forums, it's possible that by the time dual layer media is freely available and the price has come down a somewhat (probably some time between September and the end of the year), we may have seen a speed jump from 2.4x to, say, 4x, possibly requiring a new writer to get the faster speed. On a two layer write, that's a considerable time saving.
I, personally, will be letting others be the early adopters of dual layer - there again, I don't really need the added capacity. Single layer is reasonably mature, robust and relatively cheap now. I'll wait for dual layer to get faster and cheaper before getting into it myself - though if NEC do offer a free or cheap upgrade for the ND-2510A, I would probably buy it simply to maximise the value of my drive.