For he will command his angels concerning you
to guard you in all your ways.
On their hands they will bear you up,
lest you strike your foot against a stone. (Psalm 91:11-12)
In the second temptation of Jesus, we find the tempter urging Jesus to cast Himself from the pinnacle of the temple in Jerusalem thereby immediately proving that He was the Son of God (Mathew 4:6; Luke 4:10-11). By quoting this psalm, Satan suggests that God has promised to deliver Jesus from all harm should He do perform an act.
The nature of this temptation, as Dalglish rightly points out, is an ethical one; will Jesus use God's means of ambiguity, obedience, obscurity, weakness to achieve His purposes or will He use unworthy means such as power and public acclaim to accomplish this noble end? Jesus rejects Satan's temptation by responding from Deuteronomy 6:16 with its command not to tempt the Lord. The temptation that Jesus is subjected to is to use earthly means to accomplish God's purposes.
God's messengers are still tempted in this way. But Jesus' rebuttal indicates that the promises of Psalm 91 are not universally applicable; they must be interpreted in the total context of the situation and in the larger reference of Scripture. Indeed, a closer examination of the context of the verses themselves shows that the psalmist never intended them to be understood in the fashion in which the tempter used them. This is, indeed, a promise to deliver the righteous as they abide in God's shadow and walk in obedience and love towards Him (v.1, 14). The righteous can count upon God's help when they call upon Him (v. 15) with the assurance that they shall not be ultimately destroyed by those who would ambush them. The child of God can be assured of God's help in the midst of trouble but he should not presume upon God's protection if he acts autonomously or in contradiction to God's ways and means. It is enlightening to note that when it was when Jesus was on the cross that He claimed assurance from Psalms (Ps. 31:5; Luke 23:46). 
 Edward R. Dalglish, "The Use of the Book of Psalms in the New Testament." Southwestern Journal of Theology 27, 1984: 27: 38. cf. Arthur Weiser, The Psalms. SCM Press, 1962: 611.
 Dalglish: 38
 Marvin E. Tate, Psalm 51-100. Word Biblical Commentary, Word, 1990: 459.
 J. Clinton McCann, Jr. The Book of Psalms. The New Interpretator’s Bible, Vol.IV. Abingdon Press, 1996: 1048