Any chance you can give some actual details for your bug report? And it appears you are wrong:

PKGBUILD extract:
build() {
echo $startdir
echo $srcdir
echo $pkgdir
}

makechrootpkg output:
...
==> Starting build()...
/build
/build/src
/build/pkg

i supposed his PKGBUILD is faulty and he doesn't have all his patches/files to sources array and makechrootpkg is not copying his file into chroot

ohhh, my fault the problem is not the incorrect $startdir but the local src files are not copied to chroot

$ sudo makechrootpkg -r /media/data/arch32/ -- --noconfirm
==> Making package: rar 3.9.3-4 i686 (Wed Apr 21 12:53:42 UTC 2010)
==> Checking Runtime Dependencies...
==> Checking Buildtime Dependencies...
==> Retrieving Sources...
-> Downloading rarlinux-3.9.3.tar.gz...
--2010-04-21 12:53:42-- http://www.rarlab.com/rar/rarlinux-3.9.3.tar.gz
Resolving www.rarlab.com... 217.70.129.242
Connecting to www.rarlab.com|217.70.129.242|:80... connected.
HTTP request sent, awaiting response... 200 OK
Length: 833828 (814K) [application/x-gzip]
Saving to: `rarlinux-3.9.3.tar.gz.part'

100%[============================================================================================================================================>] 833,828 898K/s in 0.9s

2010-04-21 12:53:45 (898 KB/s) - `rarlinux-3.9.3.tar.gz.part' saved [833828/833828]

==> ERROR: rar.1 was not found in the build directory and is not a URL.
Build failed, check /media/data/arch32/copy/build
[andreas@neosalio rar]$ ls
PKGBUILD rar.1 rarlinux-3.9.3.tar.gz
[andreas@neosalio rar]$ ls /media/data/arch32/copy/build/
PKGBUILD src

like i said, add the file to sources array in PKGBUILD and use $srcdir/rar.1 to access it.

it is - look at http://aur.archlinux.org/packages/rar/rar/PKGBUILD

Ah... now we get some actual info about the bug...

The issue is makepkg.conf is not sourced by makechrootpkg so CARCH is unset at the point the PKGBUILD is sourced and so the source array appears to be empty.

Adding:
CARCH=$(grep '^CARCH=' /etc/makepkg.conf | cut -d= -f2)

should fix that.

With this line the problem is not solved completely.
It still didn't copy the file.

this solved the problem for me:

CARCH=$(grep '^CARCH=' "$copydir/etc/makepkg.conf" | cut -d= -f2 | sed 's|"||g')

Why not just CARCH=$(eval $(grep '^CARCH=' "$copydir/etc/makepkg.conf")) then? This should catch all cases with ', " or without them.

don't know why but i always had "i686" (with quotation marks) as value of $CARCH