There's some relevant discussion at this thread:
Will the future software be still working for G5?
"Obsolete" is such a subjective word. My father still uses a 9-year-old Power Mac 9600 with an "obsolete" 604e processor running the "dead" Mac OS 9 that works like a horse every day. It's not obsolete if it's still useful, right?
I guess you've been using your TiBook for 4 or 5 years now. If you expect your machines to last for that long (as well you should!), then it's hard to say. Certainly Intel-based machines will be more useful in 4 or 5 years. On the other hand, as someone who uses computers for a good long while, you're probably accustomed to being off the cutting edge, so maybe that won't matter so much to you. Hmm.
It all comes down to your personal expectations, I guess, and I can't offer any advice on that.
I wonder if there will be any Blu-Ray/HD-DVD drives that work with PPC-based Macs. Hmm. I hadn't even considered that before, but that's something that would be important to me. And while I think Apple has said that OS 10.5 Leopard will by universal, who knows what's in store for 10.6 or 10.7?