Short answer is no, not really.
Timing attacks can be defended against by pumping a constant stream of data through the network. That's not foolproof, however, since an entry / intermediate node could introduce lag spikes which could then be analysed.
I believe that high-latency anonymity networks (anonymous remailers) have all kinds of timing attack defences; so maybe you want to look into those.
Additionally systems such as DC-nets should be immune to timing attacks because they work in a rather different way in which there are multiple possible previous hops along the path. Of course that's not compatible with Tor though.