There are lots of wifi hotspot providers, many of whom have formed affiliate groups - e.g. BT Openzone, T-Mobile and others. I also looked at BOZII, who have aggregated a lot of providers into a single account.
This means that coverage is reasonably good, both in the UK and abroad - but it is not particularly cheap.
Another problem I have encountered is that the local wifi hotspot provider (the hotel, restaurant, coffee shop) is responsible for the actual access, and some of them are cr@p. So you won't always find a working hotspot where you expected!
3G coverage is expanding, so is another solution to consider, and prices are falling.
Belt and braces springs to mind!
One point to consider if using public wifi hotspots - your security is entirely down to you, so a decent personal firewall is essential (better than the Windows firewall).
SD