If you are using an encrypted wallet, private keys are never written unencrypted to disk.
However, the 0.4.0 release (the first to feature wallet encryption) had a bug where - upon upgrading a wallet from unencrypted to encrypted - parts of the unencrypted where not necessarily overwritten, leaving them accessible for a while.
0.4.1 and 0.5.0 fix this by first rewriting the entire wallet file upon encryption (or the first start afterwards), and invalidating the entire pool of reserve keys that was present before. This means (and has been tested), that if you start with an empty wallet, and immediately encrypt it, it will not ever use any key that was ever written to disk in unencrypted form.
Note that wallet encryption is not a silver bullet, and an attacker may have other ways of extracting the private key, such as keyloggers.
EDIT: only now I read your question correctly. The log files may contain anything that was attempted to be written to the database files themselves, and if you are using (or were using) an unencrypted wallet, that may indeed mean private keys as well. Again, if you start with an encrypted wallet in 0.5.0, no keys will be used that ever touched disk unencrypted.