For a long time, one of my biggest bugbears while commuting was the fact that I had to restart my iPhone to reconnect with my service provider's network every time the signal was lost, for example when we went underground or through a tunnel.
I queried the customer support team of my network service provider, Three, who were adamant that the problem was caused by Apple's iPhone rather than their own network, and it turned out that they were indeed right.
Since I updated my iPhone 4 to iOS 5 this morning, I can safely say that tethering is back to what it used to be when I used a dongle, with connectivity coming back flawlessly after going through a tunnel or through one of the many notorious notspots I encounter during my long journey.
Apple hasn't published the list of 200 new features or improvements it made to iOS 5, but this is by far one of the most important ones for me even if it has been and will almost certainly remain undocumented.
We've published a short, quasi-foolproof guide to updating your iPhone 4 to iOS 5. Although the initial installation procedure was marred by "error 3200" messages, the process this morning was flawless, lasting around 30 minutes only.