some GREAT news: with the server interface enhancements i've made it should be possible to PoS mine directly from a thin electrum client
Very interesting. Does it centralize the control of POS mining on the server, though?
TL;DR: no
The long answer:
I don't think so. Server can't lie to you about blocks (it's computationally infeasible, similar to 51% attack). You as a client can always verify the transactions provided by electrum server by following their history back to the coinbase transaction (though a simple electrum wallet does not do this, it just blindly trusts the server, which is ok for just a wallet - but to generate a block AND have it accepted by the network, you need to be 100% sure). Transaction hashes from a block are connected to the block's hash through the merkle tree hash, so you can verify that the list of txs provided by the server really belongs to a given block ('cause a block "can't" be faked).
(This would kinda require you as a client to keep a few full blocks somewhere for quick reference as it's impractical do re-download the same block over and over again while verifying a single tx's history. Potentially, the client can have a trusted checkpoint hardcoded and not check history before that, which would speed things up even more.)
So, if the server can't fool you into something that's wrong, it could try simply denying service to you - ie. pretending that the current network block height is just 123 while the real network is already on block 500. The solution for this is also quite simple from the client's perspective - just do a little of p2p communication with a few random peers (provided by eg. dnsseed server, or from its own cache or something) and just asking them for the number of blocks they've got (quite simple). And if the server has a lower height than the random peers agree on (needs filtering as even a peer can send you a bad number), then either it's stuck on a bad fork, lagging or lying - either way, a bad idea to use that server for PoS mining.
As for the lagging problem, the possibility that the PoS blocks you mine will be orphaned is somewhat higher than if mining with a full node - tho it's not as bad a problem for PoS, really - you can simply reuse the coinstake (ie. try again) a few times within a certain time period (and also, it's not like you don't have whole 2 months of time to collect your interest).
However, I think of it like the PoW mining pools we have today - with a bunch of electrum servers, you can choose your "PoS pool" (it can be automated) and just trust that it doesn't tell you bullshit (you just validate the chain work and txs of interest, and the standard electrum client already does that). This saves you as a client a lot of work, you don't have to connect to p2p nodes (which can even be impossible from the internet connection you're on - electrum was designed to also work over heavily restricted connections by utilizing eg. the http port 80 for its protocol and most firewalls allow at least that). You can also check with other servers whether your PoS block has gone through the network successfully. Or even utilize more than one server at a time, whatever.
Although the server does not seem to have an incetive for you to PoS mine, it's not quite true. It can either choose to serve you a lil' bit of valid data OR a thousand times more data for you to verify everything yourself (which costs the owner of that server performance and bandwidth). Even if someone didn't care about the costs and tried to fool you anyway, you wouldn't really be harmed (you can always earn your interest later via another server or even a full node if all the servers were bad for some reason). So a bad provider can't make you lose anything while he loses substantially more resources than by playing fair.
As far as your question goes, centralization of PoS mining would IMHO be something like one server with a big wallet (like a bank) where a lot of users send their money to accumulate interest. While the electrum server has absolutely zero control over your funds/interest and you can always effortlessly switch to another server for whatever reason.
whew
