Assuming you are using InfoPath 2007-style OM (InfoPath 2007 allows you to use both 2003 and 2007 object models, 2007 is preferred) you can (re)query the dataconnection by calling the Execute() method on it. For example, if your connection is called MySharePointListConnection, you can refresh it by calling the following line from your code:
this.DataConnections["MySharePointListConnection"].Execute();
I hope this answers your question.
David Airapetyan / Software Services ArchitectQdabra® Software / Streamline data gathering to turn process into knowledge