It seems that IE 7/8 draws grid incorrectly. To see the problem (attached image) you can perform the next sequence of actions:

1) Open URL http://staging.sencha.com:8080/examp...lace:basicgrid using IE7 or IE8.
2) Move your mouse on the border between any two column headers.
3) White stripe will appear instead of the border line.

If you change any column`s width, the problem will disappear.
Tested using http://staging.sencha.com and locally.