The third-party time stamp is very important to us as well. Although not perfect, I've been reasonably happy with Google as a host (Wikispaces and Blogger). Note that the date/time stamp in Blogger can be changed by the author (like presumably any blogging software). On the other hand the Wikispaces time stamp seems very robust. Of course you get further protection through redundancy, for example by discussing and re-mixing your research in the web2 sphere and various public databases.