Technically, from what I've seen from the Zoom hands that people sent me from PokerStars, the small blind is always in "Seat 2" and the big blind is always in "Seat 3" (seat numbers on PokerStars starts at one, there are a few, rare, networks, where seat numbers start from 0).
The button is always the player sitting at the "Seat 1" and there's always someone sitting at Seat 1.
Note that contrary to other hand histories (the non-Zoom ones), Zoom PokerStars hand (at least as of now), do not contain any indication saying that "button is at seat xxx".
Visually you may get the impression that the small blind is not at seat 2 but under the hood that's how it works.
Here's a sample (partial) Zoom poker hand (with players name changed):
PokerStars Zoom Hand #123456789: Hold'em No Limit ($0.02/$0.05) - 2012/03/18 7:36:05 CET [2012/03/18 2:36:05 ET]
Table 'bltable.1225924912.1225932112' 6-max
Seat 1: opponentA ($3.19 in chips)
Seat 2: opponentB ($0.69 in chips)
Seat 3: hero ($5 in chips)
Seat 4: opponentC ($17.29 in chips)
Seat 5: opponentD ($15.13 in chips)
Seat 6: opponentE ($2.50 in chips)
opponentB: posts small blind $0.02
hero: posts big blind $0.05
*** HOLE CARDS ***
Dealt to hero [Ah Ad]
In the case above the table is full. Should a deal happen on a non-empty table (I don't know if it can happen -Toby Booth- seems to say that it cannot), I'm pretty confident SB would still be at Seat 1, BB at Seat 2 and Button/dealer at Seat 1.
As to how you're seated: one would need a few hands to analyze but I'm pretty sure it's fair in that if you play, say, 600 deals of 6-max in Rush/Zoom you'll get basically 100 times at each of the six positions (just as you would if you were to play 600 regular deals at a 6-max table that would always be full). If the seating is really random and the table is always full, then it's obvious that after enough hands your positions will converge to the expected numbers.