On Thu, 2011-08-04 at 23:36 +0200, Mike Hearn wrote:
> The vending machine/detecting double spends issue was discussed by
> Satoshi in July 2010:
> 
>    https://bitcointalk.org/index.php?topic=423.msg3819#msg3819
> 
> He mentioned payment processors that could "alert the transaction is bad".
I stand with satoshi here.  No need to add more stuff to the network
protocol, a well-connected node can easily monitor the miners(/network)
for double-spends and alert whoever may need to know that the
transaction should not be accepted.  True, not everyone has the
resources to try to implement this, however the number of people who
have the resources to implement a Bitcoin storefront and not implement
this (vs those who will/do use a payment processor who handles such
things), I would think, are fairly small.
Additionally, keep in mind that many storefronts don't need to care if a
transaction confirms in 10 seconds or 1 hour.  Only digital goods and
physical purchases could benefit from such speed increases.

On Fri, 2011-08-05 at 00:10 +0200, Stefan Thomas wrote:
Since nobody else has mentioned it: There is another (more pragmatic?) 
> way to detect double spends:
> 
> 1. Connect to lots of clients
> 2a. If they all send you the same transaction -> double spend unlikely
> 2b. If some don't send you the transaction (or send a conflicting one) 
> -> double spend in progress

This is exactly what I've been suggesting this whole time.