Today there was an update for developers number 10A394. Update size is 655.3 Mb. As always, Apple's description: fixed and improved ...
Size 655.3 Mb

Two things are thrown at once into the eyes:
Already shown on WWDC is a new Expose and an obvious next mail.app update.

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What else was improved is not clear, but I am trying to look around and find everything.
Also add information about ruby, python, gcc, java.
uname -a
Darwin darkstar.unix 10.0.0b2 Darwin Kernel Version 10.0.0b2: Tue Jun 23 18:38:45 PDT 2009; root: xnu-1456.1.8 ~ 1 / RELEASE_I386 i386
ruby -v
ruby 1.8.7 (2008-08-11 patchlevel 72) [universal-darwin10.0]
python -V
Python 2.6.1
gcc -v
Using built-in specs.
Target: i686-apple-darwin10
Configured with: / var / tmp / gcc / gcc-5646 ~ 6 / src / configure --disable-checking --enable-werror --prefix = / usr --mandir = / share / man --enable-languages = c , objc, c ++, obj-c ++ --program-transform-name = / ^ [cg] [^ .-] * $ / s / $ / - 4.2 / --with-slibdir = / usr / lib --build = i686-apple-darwin10 --with-gxx-include-dir = / include / c ++ / 4.2.1 --program-prefix = i686-apple-darwin10- --host = x86_64-apple-darwin10 --target = i686- apple-darwin10
Thread model: posix
gcc version 4.2.1 (Apple Inc. build 5646)
java -version
java version "1.6.0_13"
Java (TM) SE Runtime Environment (build 1.6.0_13-b03-208)
Java HotSpot (TM) 64-Bit VM Server (build 11.3-b02-81, mixed mode)
From myself, I would like to note a stable and fast work (especially noticeable on mail.app in imap) despite its beta version, the only glitch I have is the operation of the external EMU0404 USB sound card.
If it is included in the system after switching on, it gives out wheezing. Most likely to blame the driver who can not set the normal sound purity.
PS If you have questions, see the info about the system, ask, really want to understand that in the new build
UP1dacap0 measured the performance between the two systems:
Measured my car under 10.5 and 10.6 here are the results
Results 159.06 Results 150.33
System Info System Info
Xbench Version 03/01/09 Xbench Version 03/01/09
System Version 10.5.7 (9J61) System Version 10.6 (10A394)
Physical RAM 4096 MB Physical RAM 4096 MB
Model iMac7,1 Model iMac7,1
Drive Type WDC WD15EADS-00R6B0 Drive Type WDC WD15EADS-00R6B0
CPU Test 123.77 CPU Test 140.53
GCD Loop 235.98 12.44 Mops / sec GCD Loop 234.79 12.38 Mops / sec
Floating Point Basic 112.18 2.67 Gflop / sec Floating Point Basic 113.99 2.71 Gflop / sec
vecLib FFT 91.23 3.01 Gflop / sec vecLib FFT 91.41 3.02 Gflop / sec
Floating Point Library 121.85 21.22 Mops / sec Floating Point Library 222.61 38.76 Mops / sec
Thread Test 177.82 Thread Test 173.62
Computation 163.19 3.31 Mops / sec, 4 threads Computation 166.24 3.37 Mops / sec, 4 threads
Lock Contention 195.34 8.40 Mlocks / sec, 4 thre Lock Contention 181.69 7.82 Mlocks / sec, 4 threads
Memory Test 148.34 Memory Test 159.99
System 150.22 System 165.52
Allocate 215.31 790.71 Kalloc / sec Allocate 305.41 1.12 Malloc / sec
Fill 125.18 6086.65 MB / sec Fill 128.47 6246.56 MB / sec
Copy 136.27 2814.55 MB / sec Copy 141.51 2922.92 MB / sec
Stream 146.52 Stream 154.82
Copy 136.38 2816.92 MB / sec Copy 142.74 2948.22 MB / sec
Scale 139.98 2892.00 MB / sec Scale 147.74 3052.28 MB / sec
Add 157.82 3361.83 MB / sec Add 165.79 3531.74 MB / sec
Triad 154.14 3297.41 MB / sec Triad 165.83 3547.42 MB / sec
Quartz Graphics Test 156.16 Quartz Graphics Test 157.18
Line 141.38 9.41 Klines / sec [50% alpha] Line 134.89 8.98 Klines / sec [50% alpha]
Rectangle 189.43 56.56 Krects / sec [50% alpha] Rectangle 181.14 54.08 Krects / sec [50% alpha]
Circle 151.80 12.37 Kcircles / sec [50% alpha] Circle 153.11 12.48 Kcircles / sec [50% alpha]
Bezier 150.79 3.80 Kbeziers / sec [50% alpha] Bezier 158.91 4.01 Kbeziers / sec [50% alpha]
Text 155.09 9.70 Kchars / sec Text 165.21 10.34 Kchars / sec
OpenGL Graphics Test 146.05 OpenGL Graphics Test 101.92
Spinning Squares 146.05 185.28 frames / sec Spinning Squares 101.92 129.29 frames / sec
User Interface Test 248.34 User Interface Test 216.84
Elements 248.34 1.14 Krefresh / sec Elements 216.84 995.17 refresh / sec
it can be seen that the floating-point calculations and the speed of working with RAM have increased, but the speed of the video card for OpenGL has decreased.
PS core in both cases 32 bits