Calculate total numbers based on percentages

Hey everyone. I have a spreadsheet with the following information:

Column A - Name
Column B - Allocation
Column C - Start Date
Column D - End Date

I need to work out the total number FTE (full time equivalent) resources working every month and present it in a chart. Example - if John Smith has an allocation of 0.8 and is working from 01/01/09 to 01/02/09 and Tim Bob has an allocation of 0.2 and is working from 01/01/09 to 01/02/09 that will count has one full time equivalent resource for that period.

I have attached a sample spreadsheet. Thanks in advance.

are you weighting the allocation based on days worked ?
ie a 1 allocation for a head working for only 1/2 a month is weighted to 0.5 ?

Perhaps the attached may help, I don't know as you don't indicate expected/desired results.

EDIT: error in attachment, formula in G1 should read MAX(D:D).