Does anyone know if Ron Howard has any acting projects coming up?

According to imdb.com, no.

He is currently working on a movie called Curious George (to be released in 2006) as a producer, and directing films called East of Eden (2006) (announced)
and The Da Vinci Code (2006) (pre-production)

His latest acting projects:

-"Arrested Development" (2003) TV Series (voice) (uncredited) .... Narrator
- A Beautiful Mind (2001) (uncredited) .... Man at Gouverners Ball
- Osmosis Jones (2001) (voice) .... Tom Colonic
- How the Grinch Stole Christmas (2000) (uncredited) .... Townsperson