is.matrix

That's confusing!  In what situations is x a matrix but does not have
a dim attribute?
    

That was my point. I don't find it logical that
is.matrix(a) gives FALSE but
is.matrix(t( t(a) )) gives TRUE.
  

that's a different story, because t() performs an implicit cast from
vector to matrix, so you have

t(vector) -> matrix
t(matrix) -> matrix
t(t(vector)) -> matrix

interestingly,

x = 1:2
dim(x) = 2

adds "matrix" to is(x), but it is still not is.matrix(x) (consistently
with the docs).

I also think it would be more logical that
a=c(1,1,2)
dim(a) gives 3 1 instead of NULL,
  

well, it might give 3 1 1, or 3 1 1 1, or ..., which all could be
considered logical.
i think dim(x) giving just 3 would be fine.

vQ