Is it possible to recognize conflicting 0-confirmation transactions with the Bitcoin client or other available software?
A merchant can lessen the risk of being defrauded in a race attack (on 0/unconfirmed) by:


Using an explicit list of peers to connect to (with most of the known IP addresses of miners)
Not allowing incoming connections (turn off uPnP)


this still leaves the merchant vulnerable to a 51% attack that all transactions below 6 confirmations are subject to but also to the Finney attack and another type of attack even where 2 confirmations is required (as described by vector76 here: https://bitcointalk.org/index.php?topic=36788.msg463391#msg463391 )  

What you are asking for is a solution that would be useful for merchants who can absorb some risk that comes with accepting a bitcoin transaction at 0/unconfirmed.  Currently there is only a trivial number of merchants that accept bitcoin at a retail establishment.  And thus, no solution provider -- yet.

There is an API to the service http://transactionradar.com which might make it possible to monitor for double spend attempts.

Some useful conversations related to this topic:


http://bitcointalk.org/index.php?topic=36788.0
http://bitcointalk.org/index.php?topic=39562.0
Has there ever been a successful double spend attack on the Bitcoin network? If not, is it really necessary to wait for confirmations?
http://sourceforge.net/mailarchive/message.php?msg_id=27899173
http://sourceforge.net/mailarchive/message.php?msg_id=27964471
Can bitcoins be counterfeited?
http://bitcointalk.org/index.php?topic=69520