NVIDIA's Release 340.xx GPU drivers for Windows will be the last to contain "enhancements and optimizations" for users with video cards based on architectures before Fermi. While NVIDIA will provided some extended support for 340.xx (and earlier) drivers until April 1st, 2016, they will not be able to install Release 343.xx (or later) drivers. Release 343 will only support Fermi, Kepler, and Maxwell-based GPUs.
The company has a large table on their CustHelp website filled with product models that are pining for the fjords. In short, if the model is 400-series or higher (except the GeForce 405) then it is still fully supported. If you do have the GeForce 405, or anything 300-series and prior, then GeForce Release 340.xx drivers will be the end of the line for you.
As for speculation, Fermi was the first modern GPU architecture for NVIDIA. It transitioned to standards-based (IEEE 754, etc.) data structures, introduced L1 and L2 cache, and so forth. From our DirectX 12 live blog, we also noticed that the new graphics API will, likewise, begin support at Fermi. It feels to me that NVIDIA, like Microsoft, wants to shed the transition period and work on developing a platform built around that baseline.