3
We are attempting some things in 2.0.9 - PHP5 compatibility is one thing that we hope (but do not guarantee) will be accomplished.
Existing modules will not be affected by upgrading the core - but if you plan to change to PHP5, you may find problems with some modules.
I have personally tested the 2.0.9 on PHP5 and did not find any problems in the core
Do keep in mind that it is a beta, so do not come running to us if things on your vital corporate website is not working with 2.0.9 (they *should* but I am not going to take that responsibility
)