In the next release of mojoPortal there will be a solution for you.

Right now that comes from a Resource file so it is localizable but not customizable. The resource setting is a format string like:

Signed In as: {0}

In the next version of mojoPortal you will be able to override it in layout.master by setting s property on <portal:WelcomeMessage like this:

OverrideFormat="My name is {0}", or if you only want the user name then you could put OverrideFormat="{0}"

The {0} is replaced by the user name.

Hope it helps,

Joe