In a rather odd move Microsoft has decided to discontinue all their current Zune models in favor of the new Zune HD.
According to reports by Paul Thurrott the soon to be release Zune HD will officially be the only Zune you will be able to buy from MS. Microsoft has discontinued all other Zunes, and if you want one, and from what I've seen and read you likely don't, then you need to buy now before stocks dwindle. Or do what I'd do and wait until they show up in some clearance sale.
The new Zune HD will reportedly be available September 15. Preorders of the Zune HD are available at a handful of electronic retailers, however your choices are somewhat limited. In pre-order form, you can only get a Zune HD 16 GB in the black body style. And if you preorder the 32 GB version, you can only get it in platinum.
Its uncertain as to why MS has chosen to discontinue the older models but I'd speculate that they won't to move forward and focus on the future models. After all they have been falling behind in overall sales to Apple and other companies, which make better or cheaper products.