If a bad alert is sent a special alert can be sent that disables further use of the alert system and erases all other alerts. As a result, active misuse is effectively not possible.
Good to know there is some way to counter it... although seeing 'alert key compromised' on the client probably wouldn't be the most comforting thing for most core client users.
scenario: Gavin sends a maximum sequence-number alert himself and starts spreading FUD that core keys have been compromised and to move to XT immediately
I doubt it would pay off in high numbers but people react to silly things sometimes around here
However we should think that a lot of people don't use the bitcoin core client, so *what are you talking about?
PS: *I don't want to be arrogant, but is it possible to know how many users are using bitcoin core ?
I guess I just don't see Gavin letting XT fall off the map easily and if normal methods of getting people to switch over do not work, would he be willing to go down a more malicious path? It seems like he has a lot riding on its success...
This may be relevant to your question...
http://www.reddit.com/r/Bitcoin/comments/2zt6ir/how_many_users_does_the_bitcoin_network_have/
According to getaddr.bitnodes.io, there are 6650 listening full nodes. Assuming they're all using default Bitcoin Core settings, they'll each provide 117 connection slots to the network. SPV nodes typically use 4 connection slots, and full nodes typically use 8. So the network can support a maximum of around 194,513 SPV nodes or 90,606 non-listening full nodes at one time. This is roughly the upper limit for the number of network-connected wallets that are online at any one time. (If there were more people online at once than that, people would start seeing various issues.) This doesn't include wallets that don't actually connect to the Bitcoin network, of course.
If I take my long-running listening full node and ignore connections via my Tor hidden service (which is unusual) and apparently-"fake" peers, I currently have incoming connections from 29 full nodes and 7 SPV nodes. So if I assume that my node is typical, it seems reasonable to make a very rough guess that there are at this moment (29*6650)/8 = 24,106 full nodes (6650 listening) and (7*6650)/4 = 11,637 SPV nodes (Electrum, Multibit, etc.) connected to the network.
I don't keep stats over time. I've seen more SPV nodes than that at times, but I don't know exactly when or how many.
You can get this data by giving Bitcoin Core the getpeerinfo command. Then look in "subver" to see what software that peer is running. "Satoshi" (aka Bitcoin Core) is the most common type of full node, and "bitcoinj" is the most common type of SPV node. You won't get many SPV connections unless your node has been online and stable for (I think) at least a couple weeks, though.