Problem SOLVED
The reason of FBO incomplete is due to texture consistence. When using glGenerateMipmapEXT/glGenerateMipmap, u must make sure your texture's base level contains VALID DATA; otherwise, that call fails without any error notification.
There're two possible solutions for your referrence:
Use TexImage or TexSubImage to fillup your renderable texture before glGenerateMipmap
Attach the renderable texture to a dummy FBO before glGenerateMipmap
Either way works (tested on GF8,GF9) Not tested on ATI cards.
That'd be great if anyone can test it out on ATI GPUs.
PS. Pls let me know if u have problem using these solutions.