As someone noted on the authors blog, there's a pure-PHP implementation available that's much more deployable than this solution. Shared hosts, for instance, don't generally let you pick and chose which PECL extensions you want to install - you either use what they have or you don't use them at all.
Here's the blog post mentioning the pure-PHP solution:
Jaimie Sirovich - 2009-02-20 17:01:28 - In reply to message 1 from Thomas McKinzey
This is for backups pulled to you _from_ the web server. All the web server needs is SSHd itself. Yes, I agree that it's hard to deploy on a web server, but we have it installed on a backup server over here (that we control), and it works great. The blog was about that ... not running it on a shared hosting box ;)
I haven't looked at the pure PHP implementation, but I expect it will have some nasty bugs as well ;/ Thanks for reminding me about it. I should look at it eventually.