I am trying to find all matches in a common column across multiple
worksheets and display the concatenated values of project names from
all matches in a single cell. I have been beating my head against a
wall trying to do this with array formulas to no avail.

THis is exactly what I am trying to do:

I have project schedules for several clients each on their own sheet.
Each schedule has between 1 and 25 rows (one for each project) with
columns for deadline dates for each stage of the project (copy
approval, design approval, print). On a separate sheet I have the
dreaded Excel calendar, and I would like to put a formula or UDF in
each cell that looks through each sheet for print dealine for the given
day displaying each project name that matches the date for the project
stage. Ideally the function would take a date and a project stage as
parameters and return a list of project names.

I have some programming experience with Javascript, Actionscript, and
PHP, but not with VBA.

Is this possible?

Yes, I know I should be using Microsoft Project (or similar) but that's
not an option.

Thougths? Guidance?

Michael