A few different aspects exist for the support of Java 8. The first being tolerance of the Java 8 runtime. If OpenJPA and the application are built and enhanced with Java 7 (or Java 6), then will these class files execute in a Java 8 environment? My assumption is "yes" since this would be consistent with our initial experiences with Java 7. But, some testing will be required to verify this.
The next aspect is the building of the application Entities with Java 8. Building and enhancing Java Entity class files with Java 8 looks to be hazardous... The first indication is that ASM doesn't seem to handle the Java 8 class file format. And, if ASM doesn't handle it, then I'm sure that Serp doesn't handle it either. Now, whether our use of ASM as a post-processor for Serp will suffice this time around, I have no idea.
This brings back the question of dropping Serp support altogether and going the ASM route (http://openjpa.208410.n2.nabble.com/DISCUSS-Java7-and-Serp-td6951682.html). I know there's been some interest in this in the past.
A longer-term aspect is when do we actually build OpenJPA with Java 8 and take advantage (exploit) of the new features. I think we have a long ways before we have to cross that hurdle. When we start developing JPA 2.1, we'll probably have to upgrade our build process to use Java 7. So, we're probably on the same type of cycle for Java 8 for building OpenJPA (when it's required by Java EE 8).