I'm somewhat similar to orangeapples in that I have a diverse range of interests, few (if any) of which I truly excel at, but I'm actually thinking my cutie mark, rather than being a jack as Mackheath suggested, it would more likely be some inherently positive symbol (perhaps even a Starfleet symbol, given that in addition to being an optimist, at least a third of my personality consists of pure unrefined Star Trek) that is in some way "under construction" (perhaps by way of the top part of the symbol being missing, with a few tetris blocks hovering above it and falling into place), symbolising both that I tend to do a little in a lot of areas (leaving many things unfinished) AND the fact that I'm going into engineering at the same time.
Over-complex, I know, but trying to find a less convoluted symbol that truly represents me has done little more than give me a headache. Again, it's not a jack, because while I have a tendency to achieve decent results in everything instead of excellent results in just that special something, I'm actually NOT a jack-of-all-trades at heart...
EDIT: Funnily enough, I find I have a much easier time figuring out what the cutie marks of ponyfied fictional characters might be:
Hype (from Hype: The Time Quest): A shield in the colours and bearing the symbol of Taskan IV.
Sparkster (Rocket Knight Adventures franchise): A rocket engine with a sword's blade instead of a nosecone.
Boggy B the heavily-armed worm: His signature weapons (the bazooka and the shotgun) crossed, perhaps with a stick of dyamite sitting between them like the axe in this image:
http://www.medieval-life-and-times.info/images/weapons-of-middle-ages.jpg
James T. Kirk: A single warp nacelle flaring as it prepares to make the jump to warp speeds (note that Kirk would be a special case, with the nacelle appearing first as his penchant for space exploration is revealed, and the flary glow around it only appearing later once he grows out of his, "Serious!? I was downright grim!" (Source: The episode "Shore Leave") phase from Starfleet Academy and becomes the man we know from the TV series).
Montgomery Scott: A warp core shaped like a whiskey bottle.
Spock: The Vulcan "IDIC" (Infinite Diversity in Infinite Combinations) symbol that rather than the triangle being silver and the circle behind it gold as is usual, is instead spit right down the middle, with the silver on one half and the gold on the other.