The way the segmented downloading currently works, and media in general, previewing files would be soemwhere in the range of 'technically difficult to implement' to 'completely impossible'.

MPG: Possible to preview from the beginning only, no 'fixing' required, but the first segment may be too small to play, or not even exist.

AVI: Technically possible to preview from the beginning only, but this requires sophisticated re-indexing procedures that I would bet will NEVER be incorperated into XoloX. Again, the first segment may be too small to be useful as a preview, or not even exist yet after some time of downloading.

RM/RAM: If it's not SureStream, then the first segment can theoretically be previewed, if it's fixed first, though I have yet to actually succesfully fix a broken RM file with the tools I've found to do so...

MP3: Possible to preview from anywhere in the file. This is the only media file type I know of where this is possible. MP3s are generally small though, so it's probably not worth the added bugs and programming time to implement the feature for this file type alone. In my oppinion, anyone that thinks that 320/96 is that much better than 128/44 is nuts and should just go buy the CD and rip it themselves.

Other types of files have their problems as well, or are small enough that it doesn't matter, like text files and images.