Not really, the UN can always find something on which it fails to be decried as useless. The UN was the most worthless piece of crap around on the East Timor issue until several Western countries effectively got together and decided that the friendship with ol' General Suharto wasn't going to stick around at which point the Australians and some others got together one of the more successful interventions in UN history.
An unsuccessful Korean intervention wouldn't hurt the United Nations, it would just get the USA and the USSR that much more pissed off at each other, that veto power would not be in the Security Council if the powers that be at the time of its creation did not want it to be there.
Five years from 1955 the World Health Organization is going to embark upon an (initially successful and obviously requiring HUGE logistical support) anti-malaria campaign across the globe, while the effort is a great case study in well-intentioned people armed with chemicals they do not fully understand the ecological consequences of using, it happens to be an excellent worldwide campaign by a directly UN-affiliated organization that probably won't be all that affected by a Soviet veto of Korea.
The UN will be, as always, as relevant or as irrelevant to an issue as people want it to be. Want to talk about why UNAMIR failed? Take it up with France, the UK, or the USA, not the United Nations itself (not to say people like Annan and BBG didn't drop some major balls in Rwanda).
Edit: Also, welcome back Cook.