Umm, I'd like to resurrect this post another time to say something. So I'm sorry if someone has a problem with this, but this page is still a result on Google.
I have tested Toothpaste as a TEMPORARY thermal paste. In fact, my cpu fan is sitting on toothpaste as I type this. (I can't go to the store today, needed something till then.)
I checked the temps in BIOS immediately after booting up, and my Processor was running at 50 ~ 52 C. (Normal for my slow fan)
After, I booted up windows xp and downloaded speedfan to check the temps while in use. I'm still getting temps around 54~55 C.
I wouldn't say to use it as an alternative, but if necessary, as a temporary replacement until the next day.
I read that another person tried it and advise to replace it every 45 min.
I've been running my cpu on the toothpaste for a little under an hour, while monitoring the temps, and it's still at 56 C.
The only problems are that it's temporary, and it is a little messy to clean off, but not too hard.
My processor is an Intel Pentium 4 (2.40 Ghz) running Windows XP Pro.
I don't know what temps it runs without any thermal paste.