On OS X 10.8.3 Mountain Lion, the launcher runs and the right files download, but when the launcher tries to open the new Minecraft window (the one with title "Minecraft 13w16a"), it hangs in the Mojang logo screen.

EDIT: Error message (from launcher console) appears to be
Client> java.lang.StringIndexOutOfBoundsException: String index out of range: -1
Client> at java.lang.String.substring(String.java:1937)
Client> at asm.a(SourceFile:1469)
Client> at asm.a(SourceFile:1461)
Client> at asm.k(SourceFile:1452)
Client> at asm.K(SourceFile:300)
Client> at asm.c(SourceFile:503)
Client> at net.minecraft.client.main.Main.main(SourceFile:76)