one possibility

TSC is contacting FBI for NC release, that means asking for some kind of document which can put together with your case. When this document feedback to TSC, TSC will approve the case and put this release file with your I-485 application package.

We do not know how FBI release NC, maybe some cases quick, some cases slow, just as normal NC clearance. But I wish all cases will be released and be released before April 30th 2008. Is there any possibility that some cases' NC can not be released?

This might be the reason why we can not get a confirmed answer which states before April 30th, you will certainly be approved if your case is covered by NC memo. they only tell you if you do not receive anything from TSC, call them again. So never certain untill you have your GC in hands.

Sounds NC release just like second-level NC. Hope not.