The P4 isn't anywhere near the end of it's lifecycle. It's currently at 3.4ghz, and is expected to scale up to 10ghz; we're stuck with the P4 for a LONG time further.
Many (Most) people, however, think that the Prescott SHOULD have been called the P5. Why?
1) New instruction set, SSE3. Traditionally, a new version of SSE means a new version (SSE meant P2 -> P3, SSE2 meant P3 -> P4).
2) Completely different core. The P4 has a 20 stage pipeline, the Prescott has a 31 stage pipeline. It also has different cache sizes, a new ALU mutliplication unit, a new branch predictor, etc
Why didn't Intel change the name? The prevailing theory is that it's a business decision; call the Prescott the P5 and everybody will say "Why buy a P4 when the P5 is coming out in weeks?", and Intel would lose a lot of money.
There's no question that the Prescott doesn't deserve a new name, but it's not going to get it.
And yes, while the Prescott at 3.4 is slower than the P4, keep in mind the Prescott will scale up to 5ghz. Probably about 4ghz (According to Maximum PC) is when the Prescott will start kicking ass.
Personally, though, I'll take an Athlon64 any day of the week. Cheaper, faster, better, 64-bit. Not to mention I've had AMD since the K6-2. I had a K6-2, then a Duron (Athlon), then an AthlonXP. And thanks to Alienware's choice of Intel, it looks like I'm going to have to go Intel once more
Though, come to think of it, my choice of AMD has always been because as a poor student, AMD got me more speed for my meager investments.