Sorry to bump an old thread, but I just can't help to answer this ... YES !
Yes you can brick your ADP1 so badly, but there is a way to recover though.
And I know because I have been there.
I recovered by flashing a DREAIMG.nbh for a comercial T-Mobile G1 and then I had to use the corresponding trick to root the phone, update the SPL again and so on. If anybody knows a better way to solve the problem, I wouldn't mind to know, my ADP is in big danger of having trouble again.