always in my front pocket

Wide angle, at least 24mm or wider.

Panasonic Lumix LX5 is terrific, half the price of the equivalent Leica D-Lux 5. Only $499.

$2k - I'd look at something like a Nikon D300S or Canon 7D, both are exceptional cameras.

Miles, that's a prosumer type camera. I would assume 'professional camera' means DSLR.

I did some "professional" product photography during the time when all architectural work dried up. You quickly learn the limitations of cheaper DSLR equipment.

The biggest things I would look in the body of the camera is the size of the viewfinder that you see through the eyepiece (cheap ones have tiny viewfinders, you can't even tell if you're in focus or not, useless for manual focus shooting). Cheap bodies also tend to have a slower sensor (not a big deal for architecture unless you're trying to shoot a sci-arc building that moves). Built in auto-focus assist light on cheap DSLR bodies are usually crap. Forget about capturing anything in the darkness that moves.

I would stick with either Canon or Nikon because of incredible selection of...

Camera lenses! Fixed lenses take much better photos than zoom type lenses, but they are not as versatile. For architecture my go to is a 18-50mm. Good enough for both interior and exterior work. I use 35-200mm for shooting people, weddings and porn, although at 200mm it better be super bright or you will need image stabilization inside the lens. Those lenses are usually super expensive.

If you want to do truly professional architectural photography, then you need tilt-shift type of a lens like this one. It fixes perspective issues! This lens alone will set you back $2200.

Cameras are like guns. You need to go down to B&H (9th and 34th) and hold one in your hand. And while you are at it shoot a few of the employees. The jews down there really know their cameras.

Yeah, I'll second the 'hold in your hand'.  The Nikon D7000 is technologically superior to a D300S, but being a consumer camera it just doesn't feel as good as the 300 (I have the 300).  Canon's 7D is also ergonomically superior to their much nicer 5D Mark II (imho).

If you want to spend a ton, wait for Nikon's D800, out any day!  If you want to put all the money into a body, the D700 is amazing, as is that 5D Mark II.  

A lot comes down to how much you will use it, how much you want to spend later on (lenses can easily cost more than a body).

go with film - forget digital. try to find a hassleblad SWC  (you can always get a digital back later on) or XPan - if you can find one for less then $2k.

You will find that if your purpose is to document the construction process, any decent point and shoot will work.

If your purpose is to take pictures to use as advertising and portfolio, unless you're an avid enthusiast who has put a lot of time into photography (and generally they already have a decent camera and a flash or two), a professional real-estate photographer will do a job that you could never begin to approach.

Like architecture, professional practice is best left to professionals.  Unless you have a lot of experience with photography you will not use a professional camera and get professional results.

Any full frame camera (5d mkii/ D700) with a decent wide angle (canon 16-35, for instance) is ideal for Architectural photography. If a full frame is too expensive, you can try a good crop sensor camera like the 7d with a Canon/ Sigma 10-20 lens.

Are you a professional photographer? If you were, you wouldn't be posting a camera question on an architectral forum.

What's the output? Web, print portfolio? What size prints? The camera I suggested is a solid choice with a 24mm Leica zoom lens. You might also need Photoshop ($699) and a good digital photo printer (13"x19" $6-800). All of that will fit comfortably within your budget. A "better" (i.e. professional) camera is a waste of money, especially if you are gong to be  dragging it around construction sites.

I can also vouch for the LX5. Sold my mid level SLR after seeing how great it was. Also if you are taking photos of models its small enough to get inside interesting spaces.

the olympus pen / panasonic g micro four thirds series of mirrorless cameras are really versatile and tiny while having a whole range of lenses.  I downsized from a full sized dslr, and I am much happier...since it's not a brick, I actually carry it out the house wherever i go!  the picture quality is on the same level (though they have smaller sensors, so they are 2x cropped rather than full frame), but the lens collection (since they are relatively new) is not quite as expansive, though they have a good selection from wide to tele, and adapters for any range of old lenses...yah.