Running drawnonward's code through appscript's ASTranslate tool gives me this:
#import "FNGlue/FNGlue.h"
FNApplication *finder = [FNApplication applicationWithName: @"Finder"];
FNReference *ref = [[[finder windows] at: 1] target];
FNGetCommand *cmd = [[ref get] requestedType: [ASConstant alias]];
id result = [cmd send];
The result will be an ASAlias instance; use -[ASAlias path] to get the POSIX path.
You can't do it in SB short of resorting to raw Apple event codes as that's one of the features the Apple engineers forgot/didn't bother to put into SB's less than stellar API.