Knockdowns of miRNA activity have generally used oligos targeting the miRNA guide strand. A Morpholino oligo targeting an miRNA guide strand can interfere with the activity of the miRNA. It is difficult to control for the specificity of the knockdown when using this technique alone. However, Morpholinos targeting the nucleolytic processing sites of an immature miRNA can prevent maturation of the miRNA. This allows sets of nonoverlapping Morpholino oligos targeting a primary miRNA to be used as specificity controls; if two non-overlapping oligos targeting the same miRNA produce the same phenotype, this supports the hypothesis that the phenotype is due to knocking down the activity of the targeted miRNA and not due to an off-target effect. These techniques are explored in the following paper:

Kloosterman WP, Lagendijk AK, Ketting RF, Moulton JD, Plasterk RHA. Targeted inhibition of miRNA maturation with morpholinos reveals a role for miR-375 in pancreatic islet development. PLoS Biol. 2007;5(8): e203.

Available online: http://biology.plosjournals.org/perlserv/?request=get-pdf&file=10.1371_journal.pbio.0050203-L.pdf

I've summarized the Kloosterman et al. paper on my blog:
http://blog.myspace.com/127542408