You don't think a right over information doesn't make them 'yours'?
Do you not refer to the files on your computer as your files desipite their existence only as information?
http://www.merriam-webster.com/dictionary/propertyb: the exclusive right to possess, enjoy, and dispose of a thing
I'd find it silly to make an argument that they wouldn't at least fit the common english useage of the word property. And after that it would simply be a matter of trying to determine the degree to which existing property laws apply to bitcoins. Which is more about the wording of the law then the definition of property.
Disagree?
One of the key characteristics of property seems to be that it is discernible.
E.g. I does make sense to say that a music file belongs to me, but it can not be that a song belongs to me. The former describes the physical representation of the song, the latter describes the information which is contained.
To translate this analogy to bitcoin, we may assume that the music file is something like an access key to the song. Without the file, I cannot retrieve the song. The key difference here is that a song is non-rivalrous information (because it can be consumed by more than one person at the same time), and thus strictly speaking can not be property. However, what we can say is that the music file represents a right to consume the song. Thus the argument is how that right was obtained. Bitcoin instead can be rivalrous information. Thus it fits better the description of property in the common language.