Now - If Screwtop is reading this, he might have something to say... because there are two ways to think about a starter.
1. - The one that has been talked about in this thread so far. Which is using a starter as a means to grow a small amount of yeast into an amount which is the ideal number of cells to pitch to your beer. This sort of starter is just about getting cell count. To call it a starter is a little bit misleading - its really a "growth culture"
2. - The alternate way is truly a "starter" - which is to add some wort to your pitching yeast, let it fire up and pitch it while its active. So it hits the ground running (so to speak) and you get a shorter lag time.
Now you can combine 1 & 2 by simply pitching your "growth culture" while its still in high krausen - but a lot of people don't do that because they use different conditions for growing their yeast (higher temperatures and oxygen exposure) that might mean the wort/beer that goes along with the yeast is full of undesirable flavours.
Or you can do method 1 tip off the "beer" leaving only the yeast and follow it with method 2.
Method 2 - I see no problems at all doing that with dried yeast. Re-hydrate the stuff, mix it into some wort and get it firing - dump it in. All good. Its Method 1 where you would be better off just shelling out the cash for more dried yeast than trying to grow it yourself.
TB, maybe I should've cleared up my earlier post about making starters for dried yeast. Your explanation of "method 2" is exactly what I do. Fire up the pack in a boiled and cooled wort at ferment temps, pitch when underway!
Brilliant!
Tyler
TB