In case you didn't already know, there is a workaround that allows you to resize text with the shape - if you copy an object in your OmniGraffle file as a PDF and paste it back, you can then resize it as you wish (e.g. using the shift key to constrain dimensions). However, due to the LinkBack feature, you can still edit it at a later time if you want to change any parameters.
I have an ~3MB film of this in action
here.
@ Bill - the downside of this workaround, other than it being a bit clunky is that it isn't just the text that resizes, but also any borders etc. Also, you can't alter the dimensions of the shape and maintain the proportions of the text unless you constrain the change using shift.