Is REST DELETE really idempotent?

Idempotent is about the effect of the request, not about the response code that you get.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec9.html#sec9.1.2 says:

Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N > 0 identical requests is the same as for a single request.

While you may get a different response code, the effect of sending N+1 DELETE requests to the same resource can be considered to be the same.

The important distinction is that idempotent refers to side-effects, not all-effects or responses. If you do a DELETE http://example.com/account/123 then the effect is that account 123 is now deleted from the server. That is the one and only effect, the one and only change to the state of the server. Now lets say you do the same DELETE http://example.com/account/123 request again, the server will respond differently, but its state is the same.

Its not like the DELETE request decided to change the server state in a different way because the account was missing, such as removing another account, or leaving an error log. Nay, you could call the same DELETE request a million times and you can be sure that the server is in the same state as it was the first time you called it.

Idempotence refers to the state of the system after the request has completed

In all cases (apart from the error issues - see below), the account no longer exists.

From here

"Methods can also have the property of "idempotence" in that (aside from error or expiration issues) the side-effects of N > 0 identical requests is the same as for a single request. The methods GET, HEAD, PUT and DELETE share this property. Also, the methods OPTIONS and TRACE SHOULD NOT have side effects, and so are inherently idempotent. "

The key bit there is the side-effects of N > 0 identical requests is the same as for a single request.

You would be correct to expect that the status code would be different but this does not affect the core concept of idempotency - you can send the request more than once without additional changes to the state of the server.