I applied for an extension on my 6 months visitors visa in Feb 08, my visitors visa was 'till end of March 08. I have not received a reply from Immigration, just an acknowledgement that they recd my application and they will get back to me. I should just check the status of my application online. Do I just wait, does this mean that they have approved the application and just need to process it? Can they come back and say declined? I need to know, since I have already extended my flight ticket.

To extend your stay on visitor's visa, you should file the application at least 60 days before I-94 expires.

Since your visit has been almost 6 months, at current situation, it's unlikely that you may receive an approval unless you have real emergency issues, etc. If you don't receive any response by last week of your visit, it would be best to leave by the date in I-94 because you might have problems with immigration officer next time you enter US and that might result in your visa being canceled and you're send back immediately.

even if they have my case on file, and say that I should wait for an answer max 60 days after I file? I check my application status daily, and it remains unchanged. However, they have a back-log and said that I have to wait my turn before they can reply to me. I filed 2/11/08, they rejected the application with reason no payment attached, saying I must re-submit with payment, which I have done 2/12/08.

Now I'm really concerned...

I just checked again, and this is what they say...

We suggest you file at least 45 days before your stay expires,
Or as soon as you determine your need to change status.