Assuming prop8 is an article ID, this :

<SOAP-ENV:Envelope>
<SOAP-ENV:Body>
  <ns1:Report.QueueTrended>
    <reportDescription xsi:type="ns1:reportDescription">
      <reportSuiteID xsi:type="xsd:string">[mysuite]</reportSuiteID>
      <date xsi:nil="true"/>
      <dateFrom xsi:type="xsd:string">2011-03-01</dateFrom>
      <dateTo xsi:type="xsd:string">2011-03-05</dateTo>
      <dateGranularity xsi:type="ns1:reportDefinitionDateGranularity">day</dateGranularity>
      <metrics SOAP-ENC:arrayType="ns1:reportDefinitionMetric[1]" xsi:type="ns1:reportDefinitionMetricList">
        <item xsi:type="ns1:reportDefinitionMetric">
          <id xsi:type="xsd:string">pageViews</id>
          <segment xsi:nil="true"/>
          <segmentID xsi:nil="true"/>
        </item>
      </metrics>
      <sortBy xsi:nil="true"/>
      <elements SOAP-ENC:arrayType="ns1:reportDefinitionElement[1]" xsi:type="ns1:reportDefinitionElementList">
        <item xsi:type="ns1:reportDefinitionElement">
          <id xsi:type="xsd:string">prop8</id>
          <classification xsi:nil="true"/><top xsi:nil="true"/>
          <startingWith xsi:nil="true"/>
          <search xsi:nil="true"/>
          <selected SOAP-ENC:arrayType="xsd:string[1]" xsi:type="ns1:string_array">
            <item xsi:type="xsd:string">906175</item>
          </selected>
          <expanded xsi:nil="true"/>
        </item>
      </elements>
      <locale xsi:type="ns1:reportDefinitionLocale">us</locale>
    </reportDescription>
  </ns1:Report.QueueTrended>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Produces a proper trended report for article ID 906175.

However, adding another item under <selected> like so:

<selected SOAP-ENC:arrayType="xsd:string[2]" xsi:type="ns1:string_array">
  <item xsi:type="xsd:string">906175</item>
  <item xsi:type="xsd:string">910019</item>
</selected>

produces a zero-count report. When not specifying a selection, we get the default "Top 5" trended, which is the type of report I am looking for, except for a specific set of "article IDs". What am I doing wrong?