The Olympus Pen models, along with their cousins the Panasonic G2 and GF1, and their competitors with a bigger sensor, Sony NEX3 & NEX5 and Samsung NX10, are all actually NOT DSLRs, though share some similarities with them. All of these cameras are designed with interchangeable lens capabilities, and smaller bodies than DSLRs...the Olympus and Panasonic models use a '4/3' sensor which is bigger than P&S cameras, but smaller than DSLR sensors...while the Sony and Samsung use an 'APS-C' sensor which are the same as in most DSLRs. The main difference is they do not have mirror systems inside, which provide optical viewfinder paths like DSLRs do.
If you are looking for smallish, lighter cameras with more 'P&S' style interfaces, but want better low light ability, good video capability, and interchangeable lens abilities, these cameras look like very interesting options to consider. You should handle them all to see if the designs fit in your hand and all the controls fall where you want them. Know that these camera categories are fairly new, so they don't have as many dedicated lenses you can buy - all of these cameras will work with many different DSLR lenses from almost any make of camera with various adapters, but will lose autofocus capabilities with those lenses.
They aren't quite as fast as DSLRs for shooting speed, and focus can be a little bit slower especially in lower light...and they don't have optical viewfinders, relying instead on shooting using the LCD screen, or an electronic viewfinder. The 4/3 cameras perform at higher ISOs better than any P&S camera will, but not quite as good as almost any DSLR. The Sony NEX models are actually at the very top of the low light performance pile, even compared to APS-C sensored DSLRs. The Sony models are much smaller and with a sparing, modern design, so one should handle them to see if the controls and feel would work for them. The Olympus models are a more traditional design, almost like old rangefinder cameras or larger P&S models. Panasonic's models are almost like a 'mini-DSLR' design.