I would like to buy a level 3 accurate Rhythm Blade dagger, right now. At level 4 I plan to take the feat, Superior Implement (accurate dagger). The D&D Insider character builder is allowing me to buy and equip the blade without the feat. It's showing the basic +1 that any magic item would and then if i level up and take the feat it increases the attack bonus by the +1 for being accurate. I dont want to buy a non accurate wepon because I plan to take the feat so soon. Do you think this works?
Of course you can equip the dagger; you just aren't proficient with it. I can hold a sword, that doesn't mean I know how to use it. That's what the proficiency bonus represents: training with a weapon.
Now since you're not proficient with the dagger, you don't get the proficiency bonus to melee attack rolls or ranged attack rolls.
When you become proficient with the dagger via superior weapon training, you will not gain the proficiency bonus to magical attacks.
Now regarding the "Accurate" property, yes when you gain proficiency you will gain the extra +1 from the Accurate property.
So the answer to your question is yes what you are trying to do will work
I feel like this ultimately comes down to DM fiat. As a DM if a player takes a superior implement feat I simply let them turn their current implement into the superior version of said implement. Since implements don't have proficiency bonuses (like weapons do) I'm not sure you would actually be penalized for buying an accurate implement ahead of time other than obviously not getting the further +1 from it being accurate until you had the feat to do so.