No offence but you need to see it from their perspective. Firstly discounts on the iPhone are non-existent, and what the guy is telling you about employees even is true.
But your big mistake (if I've read your post correctly) is that although you've been with them 10 years, you've failed to keep renewing with consecutive contracts. Which the guy you spoke to meant to say when he said you're 'sim-free'.
I've been with Vodafone for a decade (I added an extra O2 contract when I needed my 3G) and I've always been careful to renew every 18 months either taking a handset (to give to family or sell) or sometimes getting cash back. I was happy with them, so why not. As a result, they've always been VERY accommodating, and will "almost certainly make it worth my while" with the iPhone 4 if I choose them this time round.