To mask or not to mask, that is the question. And keep in mind, just because you can do something, doesn't mean that you should.
The issues are easier grasped by example so, to illustrate, I have created two test blogs: http://www.clutteredshelf.com and http://www.frayedcuffs.com . For the purpose of this example, http://www.frayedcuffs.com will be the good guy, http://www.clutteredshelf.com will be the evil twin.
Take a look at http://www.frayedcuffs.com The domain name frayedcuffs.com was parked on the Typepad blog using PairNIC. Do you see the short blogroll in the left colum? Click on any of those blogs, then look at the address bar of your browser. If you clicked on Rain City Guide, you saw http://www.raincityguide.com in your address bar.
Now go to http://www.clutteredshelf.com The domain name cluttershelf.com is forwarded to the Typepad blog and masked. Now click on Rain City Guide in the blogroll. What do you see in your browser's address bar? It still says http://www.clutteredshelf.com , right? Click on any of the sites in the blogroll, no matter what link you click on, your browser's address bar still reads http://www.clutteredshelf.com
Now, seriously, do you think Mr. Dustin Luther would enjoy seeing his creation displayed under the URL http://www.clutteredshelf.com ? Or Mr. Miller? Or Mr. Swann?
If it were your website or blog, would you want it displayed under someone else's URL?
Now you have grasped the single biggest issue with masking.
Try something else. Hit View -> Source (in Firefox it's View -> Page Source) and take a look at the source code. Even if you know absolutely nothing about html, you can see that the source code showing for http://www.clutteredshelf.com is only about 14 lines referencing a "frameset". Now look at the source code for http://www.frayedcuffs.com . Everything is visible, including META tags. I'll leave the significance of that to the SEO experts.
All that said --- there is one area where I think masking is a good idea: Single property websites. On a single property website you will probably not have a blogroll. You will probably have a limited number of links, and all of those links will most likely be information you created about that particular property,
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