thanks @Estel,@misterc

imho it would be a good functionality to TMO..
since it would be useful for Queries, suppose i am a new user then i would be having a lot of questions about N900,CSSU,apt-get,Easy-debian,Nitdroid.. etc
in this way i can ask my queries in each and every thread i feel might solve my problem.. without the concern of Flooding my mailbox

another reason maybe.. sometimes in a particular thread(even when that thread is not your favorite one), the current topic twists in the field of your interest so you would like to monitor and be active in that thread for a few days/hours & and then the you will Hibernate from that thread since the topic changed again..
a limited[time(1day 2days week) limited or postcount(next 5/10/20 posts) limited] subscription might come handy in this situation too

i said the same thing again, didn't i..