The major concern for most people is negative equity. This is purely a paper based figure UNLESS you are either selling or can't afford the repayments!
I'm waiting for a call today to complete my re-mortgage which puts us perilously close to going into negative equity! But, because the figures are well within our budget, even if interest rates go up by close to 3%, I have no concerns at the moment. If house prices haven't stabilised and returned to the level they were at in January when we come to the end of the deal, that's when I'll be a bit concerned! In the meantime, we'll just make sure we save as much money as we can just in case.
On the topic of saving money, I must try and get hold of Jeremy today...

If that is a rational, thought through decision (which sounds like it is in your case) its fine. problem is many people have taken out these loans (110% for gods sake - that's negative equity from day one!), without thinking what if... lose job, get sick, rates go up a lot + house prices go down etc etc. I think the banks are culpable too, but ultimately individuals need to take responsibility for themselves.
Avoid negative equity at all costs! In 1977, when living in the Netherlands, I bought a house on a 110% endowment mortgage when prices were skyrocketing. 3 months later, virtually overnight, the market changed direction, and 6 months after I bought it, the house was worth a mere 65% of what I paid for it. It took 15 years for the price to rise sufficiently to be able to sell the house, during which time my career was hampered by not having the flexibility to move anywhere else.
Basically, that's why I'm still skint, still working and driving a 10 year old Omega instead of a new Jaguar. 
I know what you're saying... Luckily with the money we're saving we'll be safe from that danger. We have no intentions of moving, unless we come into serious money, and we can afford the repayments on just one of our incomes!!

Luckily I'm in a job which is as safe as any job can be and the wife is well known in her profession and is frequently getting job offers out of the blue!
Although we have broken the golden rule and moved unsecured debt onto our mortgage we will be much better off!! Even my IFA agreed that, in our situation, it was a good move!
Still wont be driving around in a new Jag though...