it's wishful thinking. Think about it, there is no logic to this solution at all. From a technical point of view, this shouldn't fix anything. There was no Skyrim game data on the HDD prior to your initially installing Skyrim, and after playing the lag happens. Deleting the game and re-installing it (and re-downloading the day 1 patch, and/or any subsequent patches) is merely as if you were installing for the first time.
Read the whole thread, one user already states it doesn't fix it. It appears to fix it because their PS3 cooled down. So perhaps the real fix is to turn off your PS3 every couple of hours for 10-15 minutes.
It's not an issue of the system overheating, guaranteed. With normal ventilation, Skyrim would not be causing your system to overheat (especially a slim model). Skyrim is not optimized for PS3, plain and simple. It's your average port that simply is not optimized for PS3.
I have a feeling it's more related to the sheer amount of data Skyrim keeps in memory at any given time. Think about all the items that you own, everything you've killed, items stored throughout the entire world, etc, etc. Skyrim is an absolutely massive game. I'm betting there are tens of thousands of items in the game that interact with the physics engine. And it has to store that data and have it ready for whenever you happen to enter a certain area. The fact that you NEVER see a loading screen when you're exploring the (outside) world of Skyrim is mighty impressive in itself. It only makes sense that as you explore the world, gather items, kill foes, level up your character, gear, perks, and skills, that the save file become larger -- it's storing TONS of world data. See for yourself -- create a test save where you sell EVERYTHING you own, and you'll see a good drop in save file size.
Then you take the HDD into account. 5400 RPM drives are basically the slowest consumer HDDs you can buy. As the PS3 calls more textures/models/audio from the HDD and loads them into memory, that's even more system resources that's going to game data alone, and that's on top of rendering/calculating the beautiful world of Skyrim on the fly.. And if it's reading and writing to the HDD at the same time, that's even worse -- that cuts the read/write speeds in half. I'm betting it's almost entirely some type of cache/RAM/memory type issue.
If I had a spare 2.5" 7200 RPM HDD lying around, I would test that out in a second, to see if some increased bandwidth to/from the HDD would help matters at all.