Well, tigershark is the pessimist on this matter, and i´m the optimist
1st-The main argument on this matter is that we almost don´t hear anything about this matter, well, i think that speculation plays for the Russians, plus we cannot blame the Russian for keeping such important project in such secrecy.
2nd- Technology, apparently the Russian are behind in this matter, well:
a) The Russians invented stealth technology with Mr Pyotr Yakovlevich Ufimtsev, plus they have experimented plasma stealth technologies and have RAM materials, as a result doing a stealth fighter shouldn´t be that hard.
b) In terms of radars, etc, read this:
http://www.ausairpower.net/APA-2008-04.htmlThe Russians are already developing and probably testing AESA radars, and underrated version of the PAK-FA´s engine is already flying in the Su-35BM, the BM is already making use of such technologies like sensory fusion, etc.
Theres also another thing, the PAK-FA program has its origins in 2002, however it will incorporate lessons learned from programs like the Su-47 and MiG-1.44 which started in late 80´s.
Budget shouldn´t be a big problem either, people are underestimating Russia in this, they certainly went through a serious crisis, however the will to modernize its army with such projects like the new carriers, new tanks and the PAK-FA shows that money shouldn´t be a big problem.
Either way, its just like i always say, we are in 2009 and the 1st flight is scheduled for this year or for 2010, time will say if PAK-FA is just speculation or not.