Rules for breaking at inline boundaries?

Is a user agent permitted to break at inline boundaries, or should a 
lack of whitespace mean that you are dealing with a single word?

For example (assume white-space: normal):

<span>D</span><span>a</span><span>v</span><span>e</span>

Is this a single word, "Dave", or can a user agent break the line in 
between any of the inlines?  If it is considered a single word, what 
happens if some of the <span>s have borders, margins or padding?

dave
(hyatt@apple.com)