compile.options.fork = false doesn't prevent forking of the Groovy compiler. See transcript below.

/swd/prj/gradletest> gradle -version

----------------------------------------
Gradle 0.6-20081216210743+0100
----------------------------------------

Gradle buildtime: Tuesday, December 16, 2008 9:07:43 PM CET
Groovy: 1.5.6
Ant: Apache Ant version 1.7.0 compiled on December 13 2006
Ivy: 2.0.0-rc2
Java: 1.5.0_16
JVM: 1.5.0_16-133
JVM Vendor: Apple Inc.
OS Name: Mac OS X

/swd/prj/gradletest> ls
build build.gradle src

/swd/prj/gradletest> cat build.gradle
usePlugin("groovy")
sourceCompatibility = 1.5
targetCompatibility = 1.5

dependencies {
addFlatDirResolver "lib", new File("/swd/src/groovy/target/install/embeddable") // Groovy 1.6-RC-1-SNAPSHOT rev. 14452
compile ":groovy-all:1.6-RC-1-SNAPSHOT"
}

compile.options.fork = false

/swd/prj/gradletest> cat src/main/groovy/HelloWorld.groovy
class HelloWorld {
static void main(args)

}

/swd/prj/gradletest> gradle compile
:init
:resources
:compile
Exception in thread "main" java.lang.NoSuchMethodError: main // we already know that Gradle's forked compilation isn't yet compatible with Groovy 1.6, but why does it fork in the first place?

Build failed with an exception.
Run with -s or -d option to get more details. Run with -f option to get the full (very verbose) stacktrace.

Build file '/swd/prj/gradletest/build.gradle'

Execution failed for task :compile.
Cause: Forked groovyc returned error code: 1

BUILD FAILED

Total time: 4.148 secs