All subjects are scaled- some worse and some better than others. It is true that in some subjects you cannot achieve a HSC mark of 100- infact this is in most subjects. This year the only subjects where someone scored a HSC mark of 100 were Ancient History, English Extension 2, Maths extension 1 & 2, modern history, music 1, music extension, studies of religion I, french continuers, german continuers, japanese extension, latin continuers, latin extension, modern greek continuers and extension, accounting, industrial tech and distinction courses.
However all of these were scaled when calculating the UAI. For example the person who got 100% in industrial tech had their mark scaled down to 79.4 while in accounting the person who scored 100% still attained that mark after scaling. For other subjects like physics, mathematics and chemistry nobody scored a HSC mark of 100% but their scaled mark became 100.
You can see all of these on this table;
http://www.uac.edu.au/pubs/pdf/2006_table_A3.pdf
There are some subjects which you shouldn't take if you want to get a UAI over 98 because they scale extremly badly, like industrial tech.
But I took general maths and senior science which both scale fairly badly; 99 in general becomes 90.2 and 98 in science becomes 86.4 and still managed a UAI of 95.75 and a major stuff up in english.
So my advice would be that if you can achieve highly in harder subjects (advanced english, maths + extension, history, geography, physics e.t.c) do them because they scale very well.
But if you would only be getting average marks in the low to mid 70s after scaling (which would convert to the 60s as a raw mark in most subjects) then I would suggest doing subjects which don't scale as well- but make sure your achieving marks in the high 80s and low 90s because otherwise the scaling would destroy your UAI.
I only did 10 units so every subject had to count. I stuffed up english advanced getting 82, but in senior science (which scales very badly) I got 95 and that contributed to my UAI much better than english did.
So it really depends on what UAI you are hoping to obtain and what subjects you need for Uni. If the maximum you need is in the 80s or low 90s then it doesn't matter if you do subjects that scale badly. But once you start aiming for a UAI of 97+ this would become near impossible if you did a combination of badly scaled subjects (e.g. senior science, general maths, standard english, industrial tech., CaFS)- but with standard english it isn't the scaling which stuffs up the mark, its the HSC mark (the highest this year was 92 which when scaled became 92.2)
hope that helps- just remember that its alright to do some subjects which don't scale too well, just don't do all of those subjects