What stage exactly is your RP in? I misread this at first and assumed you were talking about the IC being stuck, and was about to go on a whole other bit of advice about that, but then you said something about posting character sheets. So... do you have any character sheets yet? If you have enough to at least start the game, I recommend moving right ahead, or at least telling people when you plan on having the IC ready so that people can either pick up the pace or be left behind.
Because, here's the thing: If your RP has already started, then having people not post is, understandably, very annoying, and could potentially endanger the whole RP. And if a specific person has essentially become a silent drop-out, then there could potentially be a lot of work that you, the GM, have to do in order to stop the whole RP from coming to a screeching halt. But, if you're only at the stage when people are posting character sheets? There's no reason for you to wait on every last person. Someone hasn't posted the character sheet that they said they would post? Ok. Who cares? You don't need them. They aren't holding anyone up. It's not like anyone else depends on that person's CS existing in order to continue. You can just go right ahead and start the IC without them. In fact, one could argue that doing this sort of weeds out the weak, as it were. If you don't go out of your way to hold the hands of all your stragglers, then you know that the only people left in the RP are the ones who don't need hand-holding.
In any case, though, I would say that 10 days is still fairly young. You got time. You can wait a while longer if you feel the need. :/
Also!! If you don't feel like you have enough players yet, here's an alternative solution -- contact specific players whose CS's you're waiting for. Talk to them, ask them what's up. You don't have to be all "write a CS for me right now!!", just tell them you want to know what's going on and what's keeping them. If it's writer's block, you might be able to help! You can offer ideas and suggestions to help move them along. Instead of tapping your foot impatiently while one of your players struggles to come up with ideas, you can actually get involved and help fix the problem. This also demonstrates that you're someone that these players can talk to about things -- so you don't come off as some scary authority figure that they might be afraid to talk to if they, say, decide to drop out.
I realize that I essentially just gave you two polar opposite ways of dealing with this situation. But uh... I find that, either one of these strategies can be useful, depending on what situation the RP is in. I don't know enough about your specific situation to say what action is the best, but, here are some tools for your toolbox -- hopefully that'll give you enough to figure out what to do. :P
