Is the iPad case washable ?

If you have the polyurethane Smart Cover, just use a lint roller (the sticky tape kind) to remove dust, etc. Use this on the soft inside surface. You can also use it on the outside if it's dusty.

Then, use a lightly dampened micro fiber cloth (I just dampen a corner) to clean the colored exterior. I'm hard on my Smart Cover, but it does clean up very nicely.

If it's some sort of polyurethane (and NOT leather) try the damp micro fiber cloth. I haven't found much that won't clean. If necessary, use a drop of detergent, but I've never had to.

If it's leather, test with the lightly dampened cloth on an inconspicuous area--or try a leather cleaner (again, on an inconspicuous area.) (Users here have had mixed results with leather cleaners.)