subsetting by NA

I want to know where all the NAs are in a matrix.  The data frame looks like
this:

vmc[1:5,]

       date year month day snow.new prcp      tmin snow.dep       tmax
1 01NOV1954 1954    11   1       NA   NA -14.44444       NA 12.2222222
2 02NOV1954 1954    11   2       NA   NA -13.88889       NA  2.2222222
3 03NOV1954 1954    11   3       NA   NA -16.66667       NA -1.1111111
4 04NOV1954 1954    11   4       NA   NA        NA       NA -0.5555556
5 05NOV1954 1954    11   5       NA   NA -17.22222       NA -2.7777778
       tmean yearmo
1  -1.111111 195411
2  -5.833333 195411
3  -8.888889 195411
4         NA 195411
5 -10.000000 195411


This does not work:

subset(vmc, snow.new==NA)

 [1] date     year     month    day      snow.new prcp     tmin     snow.dep
 [9] tmax     tmean    yearmo
<0 rows> (or 0-length row.names)


Because:

NA==NA

[1] NA

How can I return all rows of this data frame that contain NA in a particular
field?

Thanks,
Ben Osborne