Jon Moulton

How about heating your cell lysates just before RT-PCR?  That should blow the duplexes apart.  You might need to flash-chill to near RT.

You can detect miRNA in tissues, so it should be a similar process to detect a mimic -- unless the altered chemistry of the mimic interferes with the assay.  It might be dificult to distinguish a mimic from the background of natural miRNA.  If you are using a PCR method, the high-temperature steps should melt the miRNA duplex apart.

For a fluoresence in-situ hybridization assay, you might try something like this:
Lagendijk AK, Moulton JD, Bakkers J.  Revealing details: whole mount microRNA in situ hybridization protocol for zebrafish embryos and adult tissues. Biology Open 2012 BIO2012810; Advance Online Article April 27, 2012, doi:10.1242/bio.2012810
http://bio.biologist...n;bio.2012810v1

There is a new technique applicable for some genes -- inhibiting natural antisense transcripts to change regulation of their complementary mRNAs.

Inhibition of natural antisense transcripts in vivo results in gene-specific transcriptional upregulation.

Modarresi F, Faghihi MA, Lopez-Toledano MA, Fatemi RP, Magistri M, Brothers SP, van der Brug MP, Wahlestedt C.
Nat Biotechnol. 2012 Mar 25;30(5):453-9. doi: 10.1038/nbt.2158.
http://www.ncbi.nlm....pubmed/22446693

Another more established possibility, if the mRNA of interest is under miRNA repression, is to protect the site on the mRNA where the miRNA binds using an oligo as a target protector.

Target protectors reveal dampening and balancing of Nodal agonist and antagonist by miR-430.

Choi WY, Giraldez AJ, Schier AF.  Science. 2007 Oct 12;318(5848):271-4. Epub 2007 Aug 30.
http://www.ncbi.nlm....pubmed/17761850

This sort of effect is well documented.  If a non-native sequence loads onto RISC, you will get off-target gene modulation.

3' UTR seed matches, but not overall identity, are associated with RNAi off-targets.  Birmingham A, Anderson EM, Reynolds A, Ilsley-Tyree D, Leake D, Fedorov Y, Baskerville S, Maksimova E, Robinson K, Karpilow J, Marshall WS, Khvorova A.  Nat Methods. 2006 Mar;3(3):199-204.

Nonspecific, concentration-dependant stimulation and repression of mammalian gene expression by small interfering RNAs (siRNAs).  Persengiev SP, Zhu X and Green M.  RNA 2004; 10:12-18.

Transcriptional gene silencing by short interfering RNAs.  Kawasaki H, Taira K.  Curr Opin Molec Therap. 2005 7(2):125-31.

Small interfering RNA-induced transcriptional gene silencing in human cells.  Morris KV, Chan SWL, Jacobson SE, Looney DJ.  Science 2004 Aug 27;305(5688):1289-92. Epub 2004 Aug 5.

A rapid and sensitive assay for quantification of siRNA efficiency and specificity.  Smart N, Scambler PJ, Riley PR.  Biol Proced Online. 2005;7:1-7. Epub 2005 Jan 24.

Short interfering RNAs can induce unexpected and divergent changes in the levels of untargeted proteins in mammalian cells. Scacheri PC, Rozenblatt-Rosen O, Caplen NJ, Wolfsberg TG, Umayam L, Lee JC, Hughes CM, Shanmugam KS, Bhattacharjee A, Meyerson M, Collins FS.  Proc Natl Acad Sci U S A. 2004 Feb 17;101(7):1892-7. Epub 2004 Feb 09.

Noise amidst the silence: off-target effects of siRNAs?  Jackson AL, Linsley PS.  Trends Genet. 2004 20(11):521-4

Expression profiling reveals off-target gene regulation by RNAi. Jackson AL, Bartz SR, Schelter J, Kobayashi SV, Burchard J, Mao M, Li B, Cavet G, Linsley PS.  Nat Biotechnol. 2003 Jun;21(6):635-637.

Small RNAs with Imperfect Match to Endogenous mRNA Repress Translation: Implications for off-target activity of small inhibitory RNA in mammalian cells. Saxena S, Jonsson ZO, Dutta A. J Biol Chem.  2003 Nov 7;278(45):44312-44319.

Assuming the change is proliferation is not due to the moderate knockdown of your intended target, then perhaps it is the interaction of your knockdown reagent with an unintended RNA (an off-target knockdown) that is causing the change in proliferation.