If it isn't a keeper league, I'd definitely take Heyward for Halladay. Heyward may be a year or 2 from producing the 35+ HR, 100+ RBI people are eventually expecting out of him. Holliday is an easy write in for 25+ HR, 110 RBI, .300+ Avg, by comparison to Heyward who likely won't produce as well as Holliday this season.
If you really can do Heyward for Halladay, its a definite yes. I'd actually still say yes to Holliday for Halladay, but I'd preferably give up Heyward if it wasn't a keeper. Markakis and Bay are great fill ins.
"Chuck Norris has been known to throw a baseball 100 mph. I've been known to throw Chuck Norris 100 mph." - Brian Wilson