tteris, it's a bit of both. AGP8X, and even 4X, for the most part, provide enought bandwidth for most current graphics cards. Eventually, it will be a bottleneck, but right now, it isn't. As for the programming part, AGP and PCIe are very different; AGP is primarily 1 directional, with much less upstream bandwidth than downstream bandwidth, and current coding reflects that. PCIe, on the other hand, has equal downstream and upstream, and both are extremely fast, faster than the downstream of AGP8X. As such, PCIe allows for techniques like shuttling data back and forth from main memory to video memory, and this can let programmers have some interesting choices, like being able to store large video textures in a system without a huge video framebuffer, so long as a lot of the textures aren't used as frequently. ATI and nVidia are both planning cards that can take advantage of this, and programmers will start to use these techniques eventually.
Truth be told, PCIe offers no compelling performance advantage right now, but it's rather about the upgrade path. ATI and nVidia are doing a pretty good job of ensuring that PCIe will take over, even if it's taking longer than they'd hoped. Gateway and Dell are shipping most of their PCs with PCIe technology, even if they opt to use integrated graphics, so when all these people need better graphics to run Longhorn's Aero Glass interface (whether or not it ships with longhorn), they will be looking for PCIe. Also, the graphics firms have decided that the latest and greatest will be on PCIe first from now on, ensuring that your best value will usually be on PCIe. 6600GT was released first on PCIe and was just now released on AGP, X700 series is PCIe only, as is the refresh X850 series, the 6200 series, NV41 and R430. Even now, in the sub-$200 market, PCIe offerings perform much better than their AGP counterparts due to their newer architectures.