While neither pricing nor a release date have been announced in the U.S. yet, it appears that the HP Slate might be released by June and then be available for sale in Europe by September.
According to Clipset (yes, it's poorly translated by a website, but the big details are obvious and I can make most of it out with my own poor Spanish skills), the HP Slate would be sold the equivalent of €400 ($545). Some of the other details mentioned include the integration of an Intel Atom CPU, a built-in webcam, USB connectivity and a memory card reader. Oh, and Flash support, of course.
Not all prices are the same across countries, so we can't automatically assume that the Slate would be sold for the equivalent price stateside.