See the Ruthenium vs. Gold thread: http://bitcointalk.org/index.php?topic=494.msg4437#msg4437
Also, consider this: Bitcoin may very well have been a one-shot deal ... i.e., it benefited from being able to grow through a vulnerable stage without getting thumped by an attacker due to some coincidental timing as well as from being the first to market.
One of Bitcoin's weaknesses is that it was (and still possible could be) vulnerable if the attacker has a lot of computing power: http://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power
But at this stage, it appears that Bitcoin has weathered through its most vulnerable stages.
When Bitcoin was first "slashdotted" in June, 2010, mining was performed nearly entirely on CPUs using the classic Bitcoin client. Software for mining with GPUs had not yet been released and thus the network's hashing strength was relatively evenly distributed among nodes.
Coincidentally, once versions of the GPU-based miners did come out, the best-performing GPUs were low in availability and, as a result, were very high in cost. Thus even though there may have been chances for profit by attacking bitcoin with brute computing force, Bitcoin's path allowed it to pass through that part of its lifecycle unscathed. With each day going forward, Bitcoin's network gets even stronger and becomes less and less vulnerable over time.
A new competing currency following Bitcoin's model doesn't have the benefit of being protected by Bitcoin's current 245 Ghash/second hashing network. A new competing currency would likely not be able to follow Bitcoin's path through the early organic growth phase where it is especially vulnerable.
Looking farther down the road, the most likely scenario for launching a competing currency would be after some technology developments occur and some big money funding -- either government or corporate, has been amassed.
The funding might allow a network to be protected using the effort's direct investment in massive amounts of specialized hashing hardware using designs that have not yet left the drawing board but use components whose supply is not constrained. Such a competing currency would however be lacking the primary element that carried Bitcoin through its early days -- an enthusiastic community consisting of miners, developers, consumers and speculators.
tl;dr / summary:
Don't underestimate what has already happened that has allowed Bitcoin to become what it currently is. It will not likely be repeatable soon by a Bitcoin version 2. At a minimum, when there is a competing currency (such as version 2) that comes around, we will likely know well in advance.