As you probably dont know,Mitchel Musso has been arrested for DUI last night. 
This could mean 2 scenarios for Phineas and Ferb
If Mitchel Musso is NOT fired.
1. The show will keep going with its original plan
2. Parents possibly could ban kids from watching it, but not many.
3. However this might lead to dislike for the show in the end of 2011.
Result: Phineas and Ferb takes a 30% ratings decrease in the next 3 months.
If Mitchel Musso is Fired
1. Disney will cancel Pair of Kings.
2.Disney will replace him on Prankstars and Phineas and Ferb
Both shows goes into Downward Spiral which will lead to CANCELATIONS of both shows within 3 years And by that i mean PHINEAS AND FERB WILL BE CANCELLED IN 2013. Dan and Swampy use their Spinoff to propel the franchise back up to the top.
Pesonaly Lets Hope for a Miracle.
P.S. I know you might think he is not a main cast member and your right. HOWEVER, he has been a huge representive of the show since day 1 and heavily influenced the ratings in the beggining so BE WORRIED.