In my previous Symfony 1.1 migration step 1 I descirbed how to upgrade your old Symfony 1.0 project into Symfony 1.1 version. This is works and this is OK.
After that I upgraded project I opened it in my browser and got errors (of course, in development environment you can see it). My Symfony was dissatisfied of presence ‘web_debug: ~’ and ‘flash:Â Â Â Â ~’ lines in my ‘frontend/config/filters.yml’. I deleted these lines and everything became OK.
There are too many small issues I’ve discovered while migration process and seems like there will be more.
And another reason – I’m using many plugins from different vendors in my project so these plugins must be custom edited to work well with 1.1 – It’s a lot of work.
I made a final decision – rollback to Symfony 1.0.17 with my project and wait while almost all plugins vendors migrate to 1.2 
Because 1.1 is the transition release and for one year only.
So, I’ve got good experience with migration and have seen powerfull features of 1.1 like forms, events for example – this is right way of development, Fabien!
We’re looking forward for the 1.2.