Hi Travis,
Couple of questions. Can you add the .sid file into a blank ArcPad map? Is MrSID extension turned on in ArcPad > Options > Advanced Settings > Extensions? Does the .sid file display Projection information in Map Properties > Projection? Does the .sid file have same projection as ArcPad map document (.apm)? Does behavior occur with multiple .sid files?
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Please let us know if any of the above is helpful or if you need further assistance. If you have current ArcPad maintenance, please feel free to submit your question using Esri Support site links for phone, email, webform, or chat http://support.esri.com/en to help troubleshoot this issue. Please include ArcPad version, service pack or build information and provide screenshots of errors or dialogs which illustrate behavior you are reporting.
Thanks,
Denise