Obviously it's just one factor in the mix, but if I host a site here in the UK knowing that the majority of my users are in the U.S., how much weight should I give concerns about access speed for those users? (Currently ~70% U.S. users, ~20% UK, remainder are all over.) Trans-Atlantic hops used to be a bit of a big deal, but is it really much of a factor in today's world?
Again, I recognise this is only one factor in the overall mix, just looking to see how much weight the community thinks I should give it.
(Before you suggest it, using one of the big CDNs/clouds -- Amazon s3+ec2, Google AppEngine, etc. -- isn't an option at the moment.)
EDIT: All answers welcome; any stories of direct experience with this would be especially helpful.