Hold down the Option key when erasing using the Eraser Brush, and you’ll get a marquee that you can drag to erase a perfectly rectangular area.
Bonus Eraser Tool tip: edit the keyboard shortcuts so the Eraser uses the “E” key, and remap the Free Transform tool to Shift-E (essentially swapping the keyboard shortcuts for the two tools). Seems much more intuitive this way.
Both tips via FreeTransform

If you aren’t aware, Adobe is announcing the CS4 update the their Creative Suite design software package today. Photoshop always gets the spotlight, but us vector artists want the scoop on our baby:
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