No, the preferred form *for* modification.The only requirement on the original author (as I can determine) is that you get source code for it, not that it is in preferred form for making modification.That's perfectly acceptable. Upstream can do whatever they want. However, if upstream do not provide the preferred form for modification (ie, the unobfuscated version), Debian can not distribute it under the terms of the GPL.
If upstream own the copyright, and the obfuscated form is the only one available under the GPL, I can't see any problem - at least as far as the GPL is concerned.
Of course, if upstream have obfuscated code they don't own ... Cheers, Wol -- Anthony W. Youngman - anthony@thewolery.demon.co.uk