If there is a decrease, it's slight one. Spring, CoH about to start (I bet that will knock more people out than you would think, not quitting, but playing EQ less than before) WoW beta being as large as it is... Getting closer and closer to exam time for those still in school/college...
Quite a few factors that normally kick the EQ population down. There's a cycle, and I doubt it will be permanently broken until CoH, WoW and EQ2 are all out. EQ is beginning it's downturn in it's product life cycle, but it's going to be a fairly slow one, don't ever expect the game to fully die out. The people that do remain though will almost exclusievly (almost) people that have pleyed awhile, you will see very few new people entering the game, especially when WoW arrives (same genre).
So get to the upper 50s at least sooner (shoot for 65), as that area will be the place to find groups in 6-8 months. Below that you will almost exclusively be dealing with twinks, and they will shoot to the upper lvls fast.