Careful Stig! Technically speaking it's not exactly the same sensor. The 550D / T2i has fewer data channels, which means it can't get the information off as quickly as the 7D, which in turn is one of the factors limiting the continuous shooting rate (the others being processing and the actual physical mechanism).
But yes, many aspects of the sensor are similar or even identical to the 7D, so you'd imagine they'd be able to re-use some of the components or design for this new model. And as you say, that will keep costs down and makes sense for manufacturing.
As for why 18MPs on an entry-level model? As most of you suspect, it's driven by marketing. Most buyers still believe more MPs are better, and when faced with an 18MP camera alongisde one with, say, 15 or 12, most would go for the highest. And as Stig says, if the company can reuse some existing technology to keep costs down, then why not?
Sure, many 550D / T2is will have average lenses on the front, but the high resolution is there if you want to fit something to exploit it at a later date.
I know many are disappointed the 550D /T2i isn't tougher or faster or even more innovative, but this is the pattern we've seen from Canon for a long time and it's unlikely to change. If you're into video, it represents great value, and if the quality is even close to the 7D, then it'll be even more impressive for the money.
By the way, if any of you want to see what the EF-S 18-55mm and EF-S 18-135mm IS look like on the EOS 7D as a preview of what to expect from the 550D / T2i, check out my new EF-S 15-85mm IS review