HibernateException: 'hibernate.dialect'

Aconteceu comigo o erro:

Caused by: org.hibernate.HibernateException: 'hibernate.dialect' must be set when no Connection available

Para resolver apenas coloquei o trecho abaixo dentro da tag persistence-unit do arquivo persistence.xml:

<properties>
<property name="hibernate.dialect" value="org.hibernate.dialect.MySQLDialect"/>
<properties>

Isso foi para um banco MySQL, para outros bancos segue a lista:

RDBMS                 Dialect
DB2                   org.hibernate.dialect.DB2Dialect
DB2 AS/400            org.hibernate.dialect.DB2400Dialect
DB2 OS390             org.hibernate.dialect.DB2390Dialect
PostgreSQL            org.hibernate.dialect.PostgreSQLDialect
MySQL                 org.hibernate.dialect.MySQLDialect
MySQL with InnoDB     org.hibernate.dialect.MySQLInnoDBDialect
MySQL with MyISAM     org.hibernate.dialect.MySQLMyISAMDialect
Oracle (any version)  org.hibernate.dialect.OracleDialect
Oracle 9i             org.hibernate.dialect.Oracle9iDialect
Oracle 10g            org.hibernate.dialect.Oracle10gDialect
Sybase                org.hibernate.dialect.SybaseDialect
Sybase Anywhere       org.hibernate.dialect.SybaseAnywhereDialect
Microsoft SQL Server  org.hibernate.dialect.SQLServerDialect
SAP DB                org.hibernate.dialect.SAPDBDialect
Informix              org.hibernate.dialect.InformixDialect
HypersonicSQL         org.hibernate.dialect.HSQLDialect
Ingres                org.hibernate.dialect.IngresDialect
Progress              org.hibernate.dialect.ProgressDialect
Mckoi SQL             org.hibernate.dialect.MckoiDialect
Interbase             org.hibernate.dialect.InterbaseDialect
Pointbase             org.hibernate.dialect.PointbaseDialect
FrontBase             org.hibernate.dialect.FrontbaseDialect
Firebird              org.hibernate.dialect.FirebirdDialect

Mais detalhes em: http://docs.jboss.org/hibernate/core/3.3/reference/en/html/session-configuration.html

É isso aí!