I would do this.

Create a new sheet.

Copy the customer from 2006 to column A.  Include the single header row.

Copy the customer from 2007 to the bottom of the data in column A of that new
sheet.  Do not include the header.

Use data|filter|advanced filter to get a list of unique customers.  Put this
unique list in column B.  Look at Debra Dalgleish's site for Advanced filter
with unique records.
http://contextures.com/xladvfilter01.html#FilterUR

Delete column A (since column B has that unique list).

Put 2006 in B1 and 2007 in C1
In B2, put this formula:
=vlookup(a2,'2006'!a:b,2,false)

in C2, put this formula:
=vlookup(a2,'2007'!a:b,2,false)

(This assumes that the worksheets are named 2006 and 2007.)

Copy those formulas down as far as you need.

Select columns B:C
Edit|copy
Edit|paste special|Values

Now remove the #n/a's for the cells that didn't have a matching customer in that
year.

Select columns B:C
edit|replace
what:  #n/a
with:  (leave blank)
replace all

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This may sound like a lot of work, but once you do it a few times, you'll find
that you can do it in less than 5 minutes.