Problem:
The main problem with exchange decentralisation is lack of liquidity

The way to achieve liquidity is having centralised exchanges but this means that there are only a few points of failure (eg. mtgox getting their US account seized)

This creates a catch 22 situation in which you can't have decentralisation and liquidity at once

Solution:
An offshore exchange in a favourable and independent jurisdiction which is endorsed and run by the bitcoin foundation or other trusted people in the community which:
 - Is only accessible by other subscribing exchanges
 - Charges zero fees, but meets running costs via annual subscriptions met by member exchanges
 - Require all subscribing exchanges to post a float of both bitcoins and USD/other currencies which removes all counterparty risk of member exchanges.  This will mean that all positions taken by member exchanges are covered by their float of bitcoins and currency in the central exchange, and if they have insufficient float they will no longer be able to trade in the central exchange.  They can periodically deposit and withdraw bitcoins/USD to maintain their float as required.

Potential problems:
 - The float system means that in the case of systemic runs on bitcoins/USD, floats will be exhausted (depending on their size) and exclusion of member exchanges will occur reducing liquidity on these exchanges
 - Finding a trusted jurisdiction to run it in who will not close the accounts if they come under pressure from the US/other govts