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What constitutes a lateral brace for a beam? Does properly attached roof deck act as a continuous brace for the compression flange?

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Beam bracing can be lateral, torsional, or a combination of these.
Accordingly, a brace for a beam must restrain lateral movement of the compression flange (lateral brace), twist of the entire section (torsional brace), or a combination of these. Lateral braces are covered in Appendix 6.3.1 of the 2005 AISC Specification, and torsional braces are covered in Appendix 6.3.2. These sections address both the strength and stiffness requirements that must be met to consider a point braced.
This information also can be used to determine the answer to your second question. For the typical case, it is easy to see that roof deck spanning perpendicular to the beam and attached to  the compression flange with typical deck welds is strong and stiff  enough to be considered a lateral brace. If you have an atypical
  case, such as a heavy beam with large loads combined with a small  deck, or long-span deck, you should evaluate it using the strength and stiffness equations in Appendix 6.
Note that if you have an uplift case, the deck won’t count as a lateral brace because it is attached to the tension flange. It may provide enough restraint to be considered a torsional brace, however. Another case to evaluate further is that of deck spanning parallel to the beam.