Yea, I also still have an iphone 3G and have been waiting for an upgrade. I can vouch that you can always wait 3 months and get something better. There are many reasons this is worth getting now over a future dual-core/LTE phone.
Download speeds should be around 4-5 mbps with AT&Ts HSPA+ network. LTE will have 5-12 mbps, from what I've seen. However, the LTE will be battery hogs (black_man_x has confirmed this) while HSPA+ should be much more efficient. Also, you'll be able to keep your unlimited data plan.
The Inspire will be sporting Sense 2.0 and all the associated web features. Look them up if you are not familiar. HTC Sense appears to be the favorite skin (aside from stock).
The phone is basically a rebranded Desire HD, which means the modding community's work on that phone should be easily ported to the Inspire. You shouldn't have to wait for that initial development, it has already been done.
HTC. Best company for updates, modding (no locked down boot loaders), and general hardware reliability. And, again, the Desire HD has been out awhile and some initial problems have already been addressed (improved battery life).
The CPU has huge overclocking potential. Look up Desire HD overclocking and benchmarks if you are interested. Android has not been extensively optimized for dual core, and this processor will be snappy!
TL;DR Do eeet.