As previously mentioned, mobi is being dropped. It appears as if BeBook, and other V3 based devices, might be dropping Mobi in favor of Adobe's new DRM. Others are dropping Mobi too. If I am understanding it correctly, Mobi only lets devices use it as the only DRM available on the device. Meaning, no epub and mobi support, or mobi and anything else.
I personally just ordered a PRS-505 the other day, because I wanted epub support. Most of the ebooks I have now are in oebzip format, which is the basis for drm epub (meaning easy conversion. What is nice about that, is how easy it is to find DRM'ed books it'll read. For instance my school bookstore only supports Adobe epubs when it comes to ebooks.
I did think about a Cybook, till I saw how few formats it supported.