correct me if I am wrong, but evolution states that somewhere along the line life formed and evolved into more complex organisms....I do have that part correct, that is the theory, right? then form as many experiments that you'd like and see if an organism becomes more complex over time.
Evolution is direcitonless: it can lead to more complex organisms, or to simpler ones, depending on what's better suiting the environment. There isn't even much of an agreement on what's more complex: for one, a chimp is no less complex than a human. In fact, considering that we've been separate lineages for a few million years, we are remarkably similar with chimps: if we were bacteria, scientists would, probably, need genetic tests to even tell us apart.
In any case, if a bacteria were to develop so much as a cell nucleus, it would be orders of magnitutde more shocking and harder to accommodate within our current theories of evolution than the joint second coming and public gay marriage of Jesus Christ and the Flying Spaghetti Monster would be for you 
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A wild banana.
A banana cultivated by humans, evolved using artificial selection of many, many generations. Also, the current sweet yellow banana we enjoy today is a result of a well documented mutation.
Also, compare wolves, which are largely the same aside from a few minor differences, to the sheer variety of domesticated dogs which have been bred from those wolves. There is a staggering amount of differences in size, shape, color, fur length, fur texture, snout shape, etc.
Also,
read up on ring species.
In regards to single celled bacteria not evolving into multi-celled organisms in our observations, there's a few reasons for this.
1. A petri dish is a poor place to become bigger. There's limited space, and resources might be limited depending on whether or not the scientist is putting more stuff in there. Becoming bigger means that an organism would need more resources to sustain itself, so in the case of very limited resources being bigger may be a disadvantage. This is the reason we see
dwarfism on islands for many species through history and it's the reason why in many major extinction events we see most of the larger animals die off - limited space and limited resources favors the small.
If on the other hand the scientist is providing plentiful resources, then the environmental pressures would be rather low and as such radically changing wouldn't necessarily provide much of an advantage.
2. In the wild, there are already multi-cellular organisms. These organisms would at the very least have hundreds of millions of years of evolutionary advantage over any new multi-cellular life that evolved, and already be suited to whatever environmental niche that the new organism would be trying to take over. Faced with such odds, the multi-celled organism would inevitably go extinct faster than we humans could find it. The first few multi-celled organisms wouldn't have had this problem - all those niches would be unclaimed territory, and their only competitors would be close relatives.
In this same vein, any single celled organism we find today would have had hundreds of millions of years of evolution as a single-celled organism that lives in a world with multi-celled organisms. That's a long period of specializing, so going multi-cellular might require a few steps back which are likely to be disadvantageous, so natural selection wouldn't favor those changes. The relatives that didn't inherit the mutations to go back would have advantage over those that do, and those that do would go extinct.
