After converting your SQL Server Reporting Services reports from 2005 to 2008, you may notice a difference in the way the report looks. One possible reason for this difference is the change in the way overlapping report items are rendered.
In both SSRS versions, the message in the Output window is similar to: "[rsOverlappingReportItems] The text box ‘Textbox2’ and the text box ‘Textbox1’ overlap. Overlapping report items are not supported in all renderers." In Reporting Services 2005, these textboxes would display in an overlapped fashion, often causing the text within both textboxes to be unreadable. In Reporting Services 2008, however, the second textbox shifts, so that it appears the textboxes are vertically next to each other. This allows you to always read the text in both textboxes.
The new overlap rendering functionality is included in the "Soft Page-Break Renderers", including the Report Preview window, HTML, MHTML, Word, and Excel. Taking a look at the report in the Report Preview pane will give you a pretty good idea of how the report will look in HTML. I did not find this to be the case for the "Hard Page-Break Renderers", including Acrobat (PDF) format.
If you do have overlapping report items, the item to the "front" of the report will be rendered above the item to the "back" of the report. You can take advantage of this by right-clicking on the item and selecting either the option "Bring to Front" or "Send to Back" for the desired result.
For more information on how rendering works in SSRS 2008, I would recommend visiting this page: http://technet.microsoft.com/en-us/library/bb677573(SQL.100).aspx