NA in table with integer types

Hi -- I am having the following problem with table() when applied to
vectors of type (mode) integer.  When I use the table() command, I can
only obtain an entry in the table for NA values by using exclude=NULL.
Just issuing exclude=NA will not do it.  See below, where x is double at
first, and then integer and notice the difference.  Is this a bug or is
there something that I do not understand about the integer data type?
Thanks -- pr

------------------------------

 x <- c(1,2,3,3,NA)
 is.double(x)

[1] TRUE

 table(x,exclude=NA)

x
1 2 3
1 1 2

 table(x,exclude=NaN)

x
   1    2    3 <NA>
   1    1    2    1

 table(x,exclude=NULL)

x
   1    2    3 <NA>
   1    1    2    1

 x <- as.integer(x)
 x

[1]  1  2  3  3 NA

 is.na(x)

[1] FALSE FALSE FALSE FALSE  TRUE

 is.integer(x)

[1] TRUE

 table(x,exclude=NA)

x
1 2 3
1 1 2

 table(x,exclude=NaN)

x
1 2 3
1 1 2

 table(x,exclude=NULL)

x
   1    2    3 <NA>
   1    1    2    1

 R.version

         _
platform powerpc-apple-darwin6.8
arch     powerpc
os       darwin6.8
system   powerpc, darwin6.8
status
major    2
minor    0.1
year     2004
month    11
day      15
language R
------------------------------

==========================================================================
Paul Rathouz, Assoc. Professor       ph   773-834-1970
Dept. of Health Studies, Rm. W-264   fax  773-702-1979
University of Chicago                prathouz@health.bsd.uchicago.edu
5841 S. Maryland Ave. MC 2007
Chicago, IL  60637