If you ever need to get the GUID of a built in SharePoint field such as ‘IssueStatus’ (from the standard ‘Issue Tracking’ list) for example then you can use the SPBuiltInFieldId class.
This class contains a number of GUID objects for each type of built in SharePoint field.
To use it simply use the following code:
[code lang="csharp"]Guid issueStatus = SPBuiltInFieldId.IssueStatus;[/code]
I found this very useful, hope it helps some of you!