I am having the same issue here. The right border doesn't appear on panels and tables when viewing the report in the Html Preview pane.
Everything looks fine in Chrome and Firefox, but not in IE (tested with 6, 7, 8 and 9).
I removed the meta tag which changes the document type in IE.
The problem appears on IE6, 7 and 8. On IE 8, when I switch the document mode to IE7 and then back to IE8 in the integrated debugger (F12) the borders appear (weird behavior, since the document mode is IE8 when I run the page for the first time).
In IE9, the borders appear when the document is rendering in IE9 mode and has the same problem when running in IE8 mode (with the meta tag)
I hope this get fixed in the near future since we are looking to replace our existing reporting solution. Telerik Reporting looks really promising, but this little glitch is very annoying.