Hi!
A very good idea indeed! This could become quite handy speeding up my interface.
One question: What do you (intend to) do to ensure that accumulated results
'add up well'? 
Consider the following: you query some 10 threads, the user tags one of them
so that it doesn't macht anymore and afterwards you list the next 100 threads.
After the tagging, the number of hits decreases, so if you query
from hit number 101 to 200, you will not return the 100th (now 99th) thread at all.

Ok, one could always list all threads up to the one one is interested in but that
would make refreshing the list slower and slower when you keep scrolling down..
best,
/p