As has been pointed out, compile speed isn't necessarily CPU-bound and some compilation tasks are quicker on a slow machine with a faster drive. CPU performance is much less important than most people think.
Screen size is a more difficult issue, as so much depends on your development approach. I'm increasingly inclined to think that my large display may actually hinder my productivity, as it seems to facilitate distraction and procrastination. I seem to feel less bad about procrastinating if I have my text editor open. I'm giving very serious thought to replacing my 17" MBP with an 11" Air and a Kindle DX. A lot of writers use a full-screen text editor like WriteRoom, or even a typewriter, so there's a lot to be said for minimalist, low-distraction tools.
Using an Air as your main development machine seems odd to me - I want a large display so that I can read docs/papers and have emacs open at the same time. I also want good separation between screen and keyboard for maintaining decent posture. I hate to use a laptop as my main machine for exactly that reason. My main development machine doesn't get picked up and moved around enough (at all?) to be worth using a high end laptop. For the same price as the machine at the top of the page you could buy a decent desktop and a 13" Macbook and lunch, I guess.
Basically I have 4GB of RAM (on the latest MBP) and regularly run: Netbeans IDE, Safari (and a bunch of tabs), Firefox (ditto on the tabs), an FTP client, a Subversion client, iChat, Mail, a notes app, alarm clock, Dropbox, Evernote, terminal, one other text editor (MacVim or Textwrangler), Photoshop, MS Word, a clipboard app and sometimes VirtualBox running Windows XP (with 1GB of RAM assigned).
The killers seem to be Netbeans and VirtualBox. Once I have those both going I know I'm in for a reboot before too long.
I assume it's just a RAM issue but it is annoying that 4GB isn't "enough" (assuming I'm correct about why I have to reboot).
Did you check top? free? You should be able to see what's eating your machine after a few days of uptime.
Does shutting down all the applications and re-opening them help? Something's got to be eating all of your RAM, and it's hard to guess without seeing top/Activity Monitor.
Word. For me, every Mac has been instant-on for years. I open the lid or hit the power button (iMac), and the machine is up and running (from sleep) in a second or two.
I put the less "active" stuff on it, since there isn't a version of JollysFastVNC (fastest VNC client I've ever used) out for iPad yet, although I hear it's in progress.
And the startup time you mentioned, with the SSD - true. It's phenomenal. My Air starts up (and shuts down) about 5x faster than a Pro. It's almost instant-on.
The reality is that reading about 323k into memory is fast from both SSDs and rotational disks:
$ ls -lh `which gdm`
-rwxr-xr-x 1 root root 323K Sep 5 14:20 /usr/sbin/gdm
That said I don't need an optical drive and would love to shed 2.5 to 3 pounds. I'm sort of attached to my 1680x1050 display and 8gb of RAM though. A bit unsure if I'll trade my Pro in for an Air just yet.
1680x1050 is 1.75MP while 1440x900 is 1.3MP and 1366x768 is 1.05MP. Being frustrated with my old MacBook's resolution of 1280x800 1.02MP I'd probably have to get the 13" Air.