Damien Guard asked me an interesting question:

Is SessionFactory.OpenSession() very lightweight because you call it on every request even if you have pages that don't need it.
I would have thought the lazy construction in CurrentSession get would have been a little safer.

The answer is that it is literally not worth your time to write the lazy construction. Here is what is happening when you opening a session:

public ISession OpenSession(IDbConnection connection, bool flushBeforeCompletionEnabled, bool autoCloseSessionEnabled,
							ConnectionReleaseMode connectionReleaseMode)
{
	return
		new SessionImpl(connection, this, true, settings.CacheProvider.NextTimestamp(), interceptor,
						settings.DefaultEntityMode, flushBeforeCompletionEnabled, autoCloseSessionEnabled,
						connectionReleaseMode);
}

We new up a SessionImpl, which in turn:

internal SessionImpl(
	IDbConnection connection,
	SessionFactoryImpl factory,
	bool autoclose,
	long timestamp,
	IInterceptor interceptor,
	EntityMode entityMode,
	bool flushBeforeCompletionEnabled,
	bool autoCloseSessionEnabled,
	ConnectionReleaseMode connectionReleaseMode)
	: base(factory)
{
	using (new SessionIdLoggingContext(SessionId))
	{
		if (interceptor == null)
			throw new AssertionFailure("The interceptor can not be null.");

		rootSession = null;
		this.timestamp = timestamp;
		this.entityMode = entityMode;
		this.interceptor = interceptor;
		listeners = factory.EventListeners;
		actionQueue = new ActionQueue(this);
		persistenceContext = new StatefulPersistenceContext(this);
		this.flushBeforeCompletionEnabled = flushBeforeCompletionEnabled;
		this.autoCloseSessionEnabled = autoCloseSessionEnabled;
		this.connectionReleaseMode = connectionReleaseMode;
		connectionManager = new ConnectionManager(this, connection, connectionReleaseMode, interceptor);

		if (factory.Statistics.IsStatisticsEnabled)
		{
			factory.StatisticsImplementor.OpenSession();
		}

		if (log.IsDebugEnabled)
		{
			log.Debug(string.Format("[session-id={0}] opened session at timestamp:{1}", SessionId, timestamp));
		}

		CheckAndUpdateSessionStatus();
	}
}

Is going to setup its fields and new up some additional stuff as well. All the work that is being done is merely newing up a few objects, and that is lightning fast. There is no database involved, NHibernate will manage the database connection to ensure minimal time spent with an open database connection automatically.