based on Splash Damage's previous games a SDK is likely, but not till after release and possibly after first patch (if one is neccessary)
Mapping and modding will be difficult for all but the dedicated, It can take weeks/months just to get a playable map, it can take years to get a usable conversion/mod, but there are hundreds of dedicated people, you have to try it and figure out if you want to invest the time and energy
The same can be said for making a map for almost any game though, the custom TF2 maps that valve accepts had almost their own formal development cycle including prodigious amounts of testing by the creators who had to enlist the help of friends in the community to play through their maps.
I hope SD does release one, I'm not a mapper myself but I can imagine the possibilities if the SMART system can be implemented well on custom maps. There could be challenge and speed maps, racing mods and all kinds of interesting stuff not to mention quality maps for fighting in.