"Oh s---, I just let my Pokémon get savaged...and I didn't even win!"
In all honesty though, it would depend on how far I got in the tournament. I know that when I lost in the Top 16 at the 2009 Video Game Championships, I didn't mind - I'd gotten further than all my friends, I was awesome. However, when I went out in Round 1 this year...boy was I bummed out (didn't help that my older brother got into the Top 25 >>). However, I shook my opponent's hand, wished him luck and went over to the King of the Hill bit and pwned everyone with my Kyogre-Kingdra lead.
If I felt I lost honourably and after a good fight, I'd be happy with the loss. If it was due to luck or something else, I'd be pissed off, but I'd still shake my opponent's hand and wish them luck in the next round.