Okay, I'll be the first to say it...
I'm a bit surprised by the responses...
If this were an open carry forum and you were thinking of carrying a rubber gun to help build your confidence before carrying a real firearm I'd call you INSANE... Having that firearm on your side could potentially make you a target, and if it did, imagine the bad guys face when he pulls your rubber gun off your body?
On the other hand, since we are talking about CONCEALED carry, I absolutely don't see any problem with it...
Let's face it... The ONLY person who should EVER know you are carrying, concealed or not, is YOU...
To me it's a bit like somebody telling me not to wear one of my wife's thongs while going out... I mean, as long as I'm not bending over with my low-rise jeans, who is going to know?!? LMFAO!
In an ABSOLUTE worst case scenario... Let's say you get pulled over by a cop, or whatever... You simply tell them you are carrying it, but it's rubber since you don't have your permit yet and just wanted to get the feel of it. No different than if it was real and you did have your permit...
HOWEVER... With all that said... I'm not really sure there is any real purpose to doing it... Dummy guns don't have the heft and weight of a real firearm, especially one loaded with ammo. So even though it might give you a small taste of what it feels like, it won't actually feel like the real thing. On the other hand, if you think it will make you feel more confident, then I just can't foresee a problem with it frankly...