"Estimate effects may be unbalanced" in aov()

Can anybody explain me what does it mean "Estimated effects may be 
unbalanced", and what does it imply for the anova analysis?

Here a case example of what I get.

D<-expand.grid(A=c(0,1,2,3),B=c(0,1),C=c(0,1),res=c(runif(30,0,1)))
aov(res~A*B*C,data=D)

Call:
 aov(formula = res ~ A * B * C, data = D)

Terms:
                     A        B        C      A:B      A:C      B:C    A:B:C 
Residuals
Sum of Squares   0.00000  0.00000  0.00000  0.00000  0.00000  0.00000 
0.00000  50.93402
Deg. of Freedom        1        1        1        1        1        1 
1       472

Residual standard error: 0.3284982
Estimated effects may be unbalanced

Thanks in advance


PS: > R.Version()
$platform
[1] "powerpc-apple-darwin8.10.1"

$arch
[1] "powerpc"

$os
[1] "darwin8.10.1"

$system
[1] "powerpc, darwin8.10.1"

$status
[1] ""

$major
[1] "2"

$minor
[1] "6.0"

$year
[1] "2007"

$month
[1] "10"

$day
[1] "03"

$`svn rev`
[1] "43063"

$language
[1] "R"

$version.string
[1] "R version 2.6.0 (2007-10-03)"


--------------------------------------------------------------------
Max MANFRIN                                 Tel.: +32 (0)2 650 3168
IRIDIA - CoDE, CP 194/6                     Fax.: +32 (0)2 650 2715
Universit? Libre de Bruxelles
Av. F. D. Roosevelt, 50
1050 Brussels                             Email: mmanfrin at ulb.ac.be
BELGIUM                      WWW: http://iridia.ulb.ac.be/~mmanfrin

gpg DSA: 0xD3956AAF
gpg fingerprint: 1D69 8664 EF25 FCDE 229F FCA1 E66F EAC5 D395 6AAF
--------------------------------------------------------------------