I dont think the transition period is a reason for lack of migration 3 years is a pretty long time!. the biggest issue is lack of support in popular applications, I can't tell you the number of time's i've spoken to a hosting company about lack of PHP5 support and their answer being, "Oh we would love to move to PHP5 but our control panel and application X doesn't yet support it". This from my experience is the most common response :(

The other issue of course is BC breaks, although I can say in my experience the idea of fixing these issue's was a lot worse than actually fixing them... I worried about converting a 100,000 line PHP4 app for weeks, but in reality managed to port it to PHP5 in a weekend so I spent longer worrying more than anythig else!

We need IMO better information on how to port PHP4 app's to PHP5 detailing area's where issue's are likely to occur so that code can be converted faster.

Other than that I say roll on 8/8/08 and to PHP4 I say "So long and thanks for all the fish"!!!