There's a pretty good discussion here. Scroll down a bit for a critique of the biological species concept.
That is a good link.
However I didn't use the strict inability to interbreed which implies the inability to produce progeny or to produce only sterile hybrids in my definition. The definition I use and the one that is commonly used despite the fact that the change between one species to another is extremely gradual and thereby difficult to determine at the best of times, is the cessation of gene flow. Even though this definition is not entirely accurate when speaking of plants and definitely not with asexually reproducing species, it is useful during these debates where most are concerned with animals (Metazoans).
Whether the two groups being considered do not interbreed because of the physical limitations such as geographical location or sterile hybrids or they simply do not recognize each other as the same group, the gene flow is highly restricted. This restriction allows each group to evolve in different directions.
NOTE: John Wilkins from TalkOrigins is currently working on another FAQ further explaining the use of 'speciation' and the difference between what science means and what creationists mean and why they are wrong. As soon as it's ready I'll pass it along to PatrickHenry.
Will check it out.
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In my humble opinion, four things account for this lack of interest. First, it appears that the biological community considers this a settled question. Many researchers feel that there are already ample reports in the literature. Few of these folks have actually looked closely. To test this idea, I asked about two dozen graduate students and faculty members in the department where I'm a student whether there were examples where speciation had been observed in the literature. Everyone said that they were sure that there were. Next I asked them for citings or descriptions. Only eight of the people I talked to could give an example, only three could give more than one. But everyone was sure that there were papers in the literature.
Second, most biologists accept the idea that speciation takes a long time (relative to human life spans). Because of this we would not expect to see many speciation events actually occur. The literature has many more examples where a speciation event has been inferred from evidence than it has examples where the event is seen. This is what we would expect if speciation takes a long time.
Third, the literature contains many instances where a speciation event has been inferred. The number and quality of these cases may be evidence enough to convince most workers that speciation does occur.
Finally, most of the current interest in speciation concerns theoretical issues. Most biologists are convinced that speciation occurs. What they want to know is how it occurs. One recent book on speciation (Otte and Endler 1989) has few example of observed speciation, but a lot of discussion of theory and mechanisms.
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I don't know if I can STAND any more!