In general, you should not put a block element inside an inline element. So don't do this:

<span>
  <div>text</div>
</span>

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The browser will think you wrote invalid HTML by accident, and will sometimes reorder elements silently.

There is one notable exception: It's OK to wrap block elements in a <a> tag in HTML5 (not 4). The spec says Show archive.org snapshot :

The a element may be wrapped around entire paragraphs, lists, tables, and so forth, even entire sections, so long as there is no interactive content within (e.g. buttons or other links).

So feel free to use a single <a> link to make a series of block elements clickable:

<a href="/foo">
  <h1>title</h1>
  <p>text</p>
</a>

History of this exception

The XHTML 2 spec allowed most elements could have an href attribute. When XHTML 2 was abandoned in 2009 in favor of HTML 5, people wanted to bring over this feature to HTML 5, at least in some form. In the spirit of HTML 5, a pragmatic solution was chosen.