Hardware Acceleration and OpenGL Games Programming
OpenGL guarantees that any feature present in the core specification for the version you are using (1.1, 1.2 etc) will work, by defaulting to a built in software renderer if conformant behaviour cannot be provided by the hardware or if a software implementation would be faster. Extensions, on the other hand, should only be exported if the driver can implement them in a conformant and reasonably fast way.
While the vast majority of available OpenGL 1.1 and 1.2 drivers will accelerate almost all of the core functionality, this does raise the question of how you should check for acceleration on paths which may be implemented by a software default and thus be too slow to use in practice for a game. Note that some drivers may actually implement some operations in software even if hardware is available (since e.g. geometry on the CPU may be faster than on the card for some combinations of CPU and card with hardware accelerated geometry transformations), some operations may be implemented by "combinations" of software and hardware rendering, and the settings of other states may affect whether or not a particular operation will cause a software default. Given all this, the general approach taken in OpenGL is to write a set of speed tests which can be run at e.g. game installation or driver / hardware change to check whether particular paths which you are interested in and are not accelerated on all the machines you may be targetting run fast enough compared to some baseline to be worth using.
Note that on many systems (e.g. Windows 9X and X Windows), the speed of an operation depends on the current colour depth of the desktop. Many PC consumer cards, for example, can only provide hardware accelerated stencil buffers if the desktop colour depth is 32 bit.
A detailed discussion of how to deal with these issues and sample source code are available on the isfast page, from Allen Akin. The Glean project is a general set of open source tests which will hopefully become a single solution which many games can use.