Try setting the buffer size to one:
<appender name="AdoNetAppender_SqlServer" type="log4net.Appender.AdoNetAppender">
<connectionStringName value="DefaultConnection" />
<connectionType value="System.Data.SqlClient.SqlConnection, System.Data, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
<commandText value="INSERT INTO Log ([Date],[Thread],[Level],[Logger],[Message]) VALUES (@log_date, @thread, @log_level, @logger, @message)" />
<useTransactions value="false" />
<bufferSize value="1" />
Some more details: http://weblogs.asp.net/drnetjes/archive/2005/02/16/374780.aspx