The most reliable (perhaps only) way to do this at the moment it to start SBT with java in the JDK7 folder.
Modify your sbt launcher script; or use this one that allows you to specify Java Home (and so much more!) as command line options.
~/code/scratch/20111009 sbt -java-home /Library/Java/JavaVirtualMachines/openjdk-1.7-x86_64/Contents/Home
Starting sbt: invoke with -help for other options
[info] Loading global plugins from /Users/jason/.sbt/plugins
[info] Set current project to default-3e990a (in build file:/Users/jason/code/scratch/20111009/)
> console
[info] Compiling 1 Scala source to /Users/jason/code/scratch/20111009/target/scala-2.9.1/classes...
[info] Starting scala interpreter...
[info]
Welcome to Scala version 2.9.1.final (OpenJDK 64-Bit Server VM, Java 1.7.0-internal).
Type in expressions to have them evaluated.
Type :help for more information.
scala> java.util.Objects.equals(null, null)
res0: Boolean = true
Simply setting javaHome := Some(file("/Library/Java/JavaVirtualMachines/openjdk-1.7-x86_64/Contents/Home")) changes the Java version used to compile and fork processes, but does not change the version of the Java standard library on the classpath, nor the version used to run tests, which are always run the the same JVM as SBT.