The NEC is not in effect untill it is addopted by the local jurisdiction. Arround here that may be a year after it's issued.
Not only do they take their time they may take exceptions to certain articals and issue amendments.
The 2007 New York City Electric Code is based on the 2005 NEC. Some other New York jurisdictions still haven't accepted the 2005 code. Check your AHJ before buying the 2008 NEC.
Here in California we just finished adopting the 2005 NEC in June (I think). The changes are only now (October) beginning to take effect in the field as far as AHJ implementation. I did a seminar last May on the biggest change affecting my niche (Article 409) and although most people had heard it was coming, no AHJs were looking for it's requirements until probably September. This followed adoption of the 2002 code in 2004 I believe, so to follow the same pattern I'll wait to buy my copy until Sept of 2010.
Different requirements apply to different locations. The new NEC 2008 is available. Consider the 2008 NEC where projects require the current edition. My clients require the current edition of the codes that apply when the project is funded.
You may actually find yourself in a very good position regarding which edition's requirements to follow. You may be able to use the 2008 version for articles where it is particularly helpful for a given situation, and be able to get a variance from the AHJ to allow it, before it has actually been adopted.
Don't use it before you know it very well though.
As far as the "NEC" (National Exception Catalog) goes, newer is seldom better. Newer will help you to reduce the high costs of true safety factors and "overbuilding" though. But rather than listen to a grumpy guy like me, go study the editions since the mid 1970s. No, the electrical work performed these days is not better, let alone anywhere near the page-count multiple since then.
