I have tried to start a 2nd bitcoin instance on Win7 (v0.3.24-beta of the bitcoin client); and the application silently terminates. Instead of telling me what has gone wrong, it dumps the following into debug.log

Bitcoin version 0.3.24-beta
OS version Windows NT 6.1 (build 7601, Service Pack 1), 64-bit edition
System default language is 80 French_France.1252
Language file locale/fr_FR/LC_MESSAGES/bitcoin.mo (French)
Default data directory C:\Users\JoannesVermorel\AppData\Roaming\Bitcoin
Existing instance found

It would probably better to have a message box telling Bitcoin client is already running, please close this instance first to open a new one.

My 2cts on this subject,

Best regards,
Joannes Vermorel