hello kheibar you should not be  panic bear in mind  the whole process normally takes up 6 months so as long as your application has not pass that six months slot time its ok, and you have to understand UKBA are processing a millions and millions of applications every day.
when it comes how long it takes every one is different from the other one for example I have sent my application on 24th of August they have received on 28 of August and I have  received the biometric enrolment  on 19 October 2012 although they have sent it 12 of October, it took them one month and half you can call me a lucky person!! to get that biometric enrollment letters within one month and half but it also depends upon the person who is handling the case.
you have  repeated many times about acknowledgement letter I have made a research and what I found out is from 29 of February 2012 that is the time when they (THE UKBA)  have started this biometric enrollment policy UKBA have stopped sending the acknowledgement letters and I reckon the reason for the volume of applications they are receiving every week is more than them!!! on my case the first page of the biometric enrollment letter was elucidating  the acknowledgement that they have received my documents.
on the other hand if you are so curious and desperate to know whether your documents are with them just you can track from the Royal Mail website 
http://track2.royalmail.com/portal/rm/t ... gear=track
 
if how ever you are not happy for their slow process you can complain by sending an  email to  their complaint team for this email address  
UKBACust1568@homeoffice.gsi.gov.uk
bear in mind their response may take up to 20 days.
I wish every one for good luck and I am still waiting the ILR ID card (not more A4 size sheet) kk