Are there any studies into the size of the blockchain scaling over time? After watching some of one of the 28c3 talks, It got me thinking about just how scalable Bitcoin really is.

With the size of the blockchain currently, given the number of transactions/users, will the size of the blockchain be manageable as Bitcoin adoption increases?

As an example (no answers about "this will never happen" please), if Bitcoin was as large as a company like VISA, hundreds/thousands of megabytes would have to be transmitted over the network daily and stored in the blockchain. This is not including other things being stored in the blockchain that essentially shouldn't be (Eligius putting prayers in the blockchain for example).

This would mean that most casual users would need to use a thin client as storing the blockchain and keeping it up to date would require super fast internet and masses of hard drive space. (imagine Visa's 92 billion transactions per year)

Are there any studies into the scalability of the blockchain? Or is the assumption that technology will be so advanced by the time of this ever occurring that it will not be an issue? (which doesn't sound like the greatest conclusion in my opinion)

Also, quoting the wiki:


  At very high transaction rates each block can be over a gigabyte in size. These blocks must be stored somewhere. Whilst for speed it'd be ideal to store the block chain entirely in RAM, for cheapness storing only the hot parts in RAM and the rest on disk is the way to go. A 3 terabyte hard disk costs less than $200 today and will be cheaper still in future, so you'd need one such disk for every 21 days of operation (at 1gb per block).


3 TB every 21 days? 52 TB a year for the blockchain alone? Is that REALLY scalable?
Satoshi Nakamoto addressed this issue in the original crypto forum thread.


  Long before the network gets anywhere near as large as that, it would be safe 
  for users to use Simplified Payment Verification (section 8) to check for 
  double spending, which only requires having the chain of block headers, or 
  about 12KB per day.  Only people trying to create new coins would need to run 
  network nodes.  At first, most users would run network nodes, but as the 
  network grows beyond a certain point, it would be left more and more to 
  specialists with server farms of specialized hardware.  A server farm would 
  only need to have one node on the network and the rest of the LAN connects with 
  that one node.
  
  The bandwidth might not be as prohibitive as you think.  A typical transaction 
  would be about 400 bytes (ECC is nicely compact).  Each transaction has to be 
  broadcast twice, so lets say 1KB per transaction.  Visa processed 37 billion 
  transactions in FY2008, or an average of 100 million transactions per day.
  That many transactions would take 100GB of bandwidth, or the size of 12 DVD or 
  2 HD quality movies, or about $18 worth of bandwidth at current prices.
  
  If the network were to get that big, it would take several years, and by then, 
  sending 2 HD movies over the Internet would probably not seem like a big deal.