How is a DIY install of OS X (purchased) on a Dell not legal?
Contrary to your assertion, you would not have purchased OS X because Apple does not sell the software of OS X. You have purchased permission, a license, to use Apple's property, OS X, under certain specified circumstances, one of which is that the upgrade copy of OS X accompanying that license can only be used on a single computer manufactured by Apple that already has a previous installation of an earlier version of OS X installed on it.
A computer manufactured by Dell does not meet that criteria specified in the license to allow you to legally install it, DIY or otherwise, on that computer because it is neither manufactured by Apple nor does it have a legally licensed earlier version of OS X installed on it to be upgraded. Ergo, it is not legal to install... and doing so, voids your license. Apple would be within its rights to brick your computer and remove its property from your computer and erase any copies in your possession.
Contrary to what you may read from people who wish to believe that software is "sold," and that the "right of first sale" applies to software, it doesn't—because the software is not sold. The courts at both trial and appellate levels have upheld this interpretation of software user licensing. It is more akin to a rental in perpetuity, with title remaining with the copyright holder. Your rights, what you are allowed to do with it, are limited to what the owner of the copyright allows you to do until the copyright monopoly expires.
Some people point to the Lexmark ink case as though it were definitive... it's not because the ink is not software and was not copyrighted. Others point to the Autocad case... but that involved a case where a man bought the software, unopened at a garage sale, never installed it or read the Software License Agreement, and merely resold it to an end user through eBay, again, not germane to the issue at hand. Another involved the gift to Lauren Bacall of a copy of a movie she starred in without the usual licensing... but since there was no license, it was a sale and held so, again, not germane.
All of these cases were cited by Psystar in the recent Psystar copyright infringement case, justifying their installation of "purchased OS X" on non-Apple hardware, and were all shot down by the judge as irrelevant because they would still have been in violation of the Apple software license agreement... just as you would be if you installed OS X on the Dell.
To give you an idea of just HOW illegal it is, Psystar Corporation has just agreed in a settlement after being found by Summary Judgement by a Federal Judge to have infringed Apple's copyrights AND violated the DMCA on OS X, to pay Apple $3,476.56 for each and every one of the 768 Psystar Open computers they made and sold for between $550 and $1500 each with OS X Leopard and Snow Leopard installed on them over the space of about 18 months.
Statutorily, the Judge could have imposed fines far greater... some copyright law experts estimated up to $70,000 per computer. Apple knew that given the state of Psystar's finances, there is very little chance of even getting the agreed settlement—merely stopping the infringement will probably have be enough satisfaction.
That settlement is irrespective of what Judge Alsup might levy as criminal fines for their willful violation of the provisions of the DMCA, in that they deliberately used countermeasures to get around encryption to install OS X on their computers... something the DMCA specifically prohibits.