I've built quite a few Zipp wheels with Ti spokes (bladed) and alloy nipples, and some conventional wheelsets with them as well. I dosed both the spokes and nipples' threads with liberal doses of Ti Prep and antisieze compound, and siezures were rare.

Due to the low mass of Ti, the tone of a Ti spoke will be higher at a given tension than a steel spoke, resulting in some pretty darn high pitches when a fully-tensioned wheel's spokes are plucked. *shrug* That's physics for you. If people are relying on tone, the Ti spokes could fool them into stopping early.

One factor I haven't seen mentioned here yet is the increase in frontal area (and therefore air drag) of a round 2.0mm Ti spoke. Butted 14/15 stainless in back and 15/16's up front might be worth a weight penalty if you cruise at high speeds a lot, since they'll have a lower frontal area. On a 'cross bike, that won't be so much of a factor perhaps. Something to think about, anyway.