When it comes to drivers, people have many different interpretations of what "beta" driver is. In the days when driver "leaks" were happening on weekly basis, I used to attach term "beta" to them to give people warning that if they use them - its only at their own risk. I've done that with both nvidia and ati drivers.
Extremly buggy drivers are also seen as "beta" even though they might not be or not intended to be by the software developers. There has been few of these in last few years.
Both nvidia and ati has also posted drivers on their official websites attaching term BETA to them - warning people they either might get no support if they use them or warning people they might not be stable and have unresolved issues in them. I don't know what they mean by BETA in this case - perhaps they mean they are not finished yet.
Of course, we can also go all fanboy here and say that all nvidia drivers since 40.41 has been BETA

or all ATi drivers before catalyst 2.0 or something
As to WHQL - I don't belive Microsoft has stressed enough the advantages of such drivers or have pushed companies to release WHQL drivers more often. NVIDIA certainly has not bothered much with WHQL recently, I really don't know why...

Personally, I found that WHQL drivers do not always perform better then non-whql ones. They might be more stable - but a lot of people would prefer occasional error here and there for more speed (at least in the old days)...What would be best, is to see WHQL drivers that do not sacrifice extra performace for stability - and vice versa when it comes to non-whql drivers. I wonder if BOTH ATi and NVIDIA has got the balance just right...