Amazon is still having a hard time meeting the at-the-moment-nebulous demand for the Kindle. A pal of mine ordered his on March 24th, but as of today it still hasn't shipped. Digitimes reported in January that PVI, the company that makes the Kindle's ePaper display, is having trouble meeting demand. I suspect that is still the snag.
Keep in mind this doesn't necessarily mean the Kindle is doing huge numbers overall. I requested sales about the device from Amazon several weeks ago and was met with silence (from a contact that had otherwise been happy to talk), so at this point it's possible that the overall unit sales could be low. (Or high! But usually when they're high you get some sort of response.) And they'll remain low if Amazon can't start getting Kindles into the hands of the unknown number of customers who want one.
Update: That same pal just got this email: "We now have estimated delivery dates for the Kindle order you placed on 3/24/08, #[redacted]. We are now estimating that your Kindle will arrive between 4/20/2008 and 4/27/2008."