<p>I have monthly sales figures stored in separate sheets.  I would like to create a plot of sales for multiple products per month.  Each product would be represented in a different colored line on the same chart with each month running along the x axis.</p>

<p>What is the best way to create a single line chart that pulls from the same relative cells on multiple sheets?</p>

Use the Chart Wizard.
On Step 2 of 4, there is a tab labeled "Series". There are 3 fields and a list box on this tab. The list box shows the different series you are already including on the chart. Each series has both a "Name" field and a "Values" field that is specific to that series. The final field is the "Category (X) axis labels" field, which is common to all series.
Click on the "Add" button below the list box. This will add a blank series to your list box. Notice that the values for "Name" and for "Values" change when you highlight a series in the list box.
Select your new series.
There is an icon in each field on the right side. This icon allows you to select cells in the workbook to pull the data from. When you click it, the Wizard temporarily hides itself (except for the field you are working in) allowing you to interact with the workbook.
Select the appropriate sheet in the workbook and then select the fields with the data you want to show in the chart. The button on the right of the field can be clicked to unhide the wizard.
Hope that helps.
EDIT: The above applies to 2003 and before. For 2007, when the chart is selected, you should be able to do a similar action using the "Select Data" option on the "Design" tab of the ribbon. This opens up a dialog box listing the Series for the chart. You can select the series just as you could in Excel 2003, but you must use the "Add" and "Edit" buttons to define custom series.