Short answer to the smoking gun question. Yes. (I don't have an answer to the alternative question.)
Long answer to the smoking gun question. The only alternative to yes, is to believe that the White House deceived, not the world, but the British government only, and in a very peculiar way; contriving, for reasons more or less unimaginable in our present universe, to _falsely_ convince the British government that we intended to go to war come hell or high water and moreover that we were gonna manufacture bogus intelligence to do it.
In other words, to think that the Downing Street Memo is not a smoking gun, you have to believe something really, um, unusual--that the White House snookered the Brits into thinking that we intended to lie our way into a war, when in fact it was just a gag, ha ha, Tony, you sure fell for that one. A real knee-slapper.
Is there a third possible explanation? If we wait for our press, clear-thinking or otherwise, to provide it, we may wait a long time. So far their 3-legged stool of saying nothing, or saying it's old news, or saying gosh, we don't see any smoke, seems to be satisfy them. A wobbly piece of work.
And now even wobblier, with another leaked document, a British Cabinet Office briefing that says Tony Blair had agreed to the war in spring of 2002 at a meeting with Bush in Crawford. (One imagines excited barks.)
According to the London Times, the leaked Cabinet Office briefing paper "noted that since regime change was illegal it was 'necessary to create the conditions' which would make it legal."
This task seems not to have been entirely successful.