You actually can tell you're pregnant by temping, at least according to the awesomely comprehensive book Taking Charge of Your Fertility.
Your temperature goes up the day after ovulation (1 day past ovulation, or 1DPO). If it stays up for the next 18 days you are most likely pregnant. It will drop back down for your period, so if you see it dropping you are likely not pregnant.
One dip in the middle of the time after ovulation (the two week wait) isn't a big deal, if it goes back up it's still considered up. In fact that dip might be an implantation dip, which 10% or so of women who aren't pregnant get and 20% of women who are. It happens right around when implantation might occur.