Just emailed over clockers and got this reply:
"Yes it will work straight out of the box no flash required."
So who knows, would it be worth sticking the E8400 in straight away or should i play it safe and update the BIOS first.
Also just had a thought, does anyone have any recommendations for a good hard drive, have around £50 and need 250 to 320GB, thanks.
I'm not going to dispute what OCUK say, but I'd still like to see someone who has done it.
Maybe you could be the guinea pig

2 posts here on ASUS boards (tho not the P5E)
I just popped in my shiny noo E8400, the bios saw the new cpu, said I had to hit F2 to have default values, I hit F2, doh!! I forgot to plug my keyboard back in. Turn pc off, plug keyboard back in, reboot and nothing!!! Lights on but no-one is home. Friggin' Asus junk, I get this nightmare everytime I change cpu's, and thats too often. Might have to get a Gigabyte again.
Its this one shot at updating the BIOS thats worrying.
If I find anyone else whos done it on my travels, I'll edit this.
Hard drives is a personal choice, but I'd be looking here if I had that to spend.