The hard part to replicate about bitcoin is the community. It took two years to build up the community, and I think that it would be difficult to replicate that just by tweaking the code a bit and starting a parrallel blockchain.
The community is relatively small and two years is not all that long. See what Facebook did to MySpace as an example. And MySpace was significantly more entrenched in the mainstream public than Bitcoin currently is but that didn't make much of a difference.
That said, anyone is welcome to try it. If it has features that make it superior to bitcoin, it would probably eventually win. However, they must be features that are obviously superior to a large percentage of the Internet using population.
I agree.
I don't really think that is going to happen, myself.
If Bitcoin does become even moderately successful, I don't see how it would not happen. Where profit can be had, competition will grow. I find it hard to believe that an open source project that has no protection (via patents, trademarks, etc.) would not garner competition. The barriers to entry are just too low.
The two year headstart is also a bit of a misnomer. Growth was rather stagnant for the majority of that time, with a high rate of growth over more recent periods. Really Bitcoins major advantage is being first to market and having a small geek community base. First to market doesn't matter if the model can be copied and the Bitcoin community is nominal in scale. I'd be curious to hear of any serious barriers to entry by a competing crypto-currency because I don't see any.
noooooo... impossssssibbbllllee... 
