First results (intrepid) for rebuilding packages with reverse build depend on fpc:
gearhead builds normal in intrepid
hedgewars builds normal in intrepid
imapcopy builds normal in intrepid
libhdate builds normal in intrepid
m-tx builds normal in intrepid

lazarus fails to build in intrepid with:
gtkproc.pp(674,11) Error: Forward declaration not solved "MergeClipping(TDeviceContext, PGdkGC, LongInt, LongInt, LongInt, LongInt, PGdkBitMap, LongInt, LongInt,var PGdkBitMap)"

The following commands didn't show any change except the own I made in the changelog:
$ debdiff <original>.dsc <new-build>.dsc

and the following command showed:
$ debdiff <original>.deb <new-build>.deb

===========
$ debdiff gearhead_1.100-1_i386.deb ../../intepid/gearhead_1.100-1ubuntu0.1~ppa1i_i386.deb 
File lists identical (after any substitutions)  

Control files: lines which differ (wdiff format)
------------------------------------------------
Installed-Size: [-972-] {+948+}                 
Maintainer: [-Ubuntu MOTU Developers <email address hidden>-]
[-Original-Maintainer:-] Kari Pahula <email address hidden>               
Version: [-1.100-1-] {+1.100-1ubuntu0.1~ppa1i+} 

===========
$ debdiff imapcopy_1.01+20060420-1_i386.deb ../../intepid/imapcopy_1.01+20060420-1ubuntu0.1~ppa1i_i386.deb                 
File lists identical (after any substitutions)                                  

Control files: lines which differ (wdiff format)
------------------------------------------------
Depends: libc6 (>= [-2.4-1)-] {+2.0)+}          
Installed-Size: [-500-] {+556+}                 
Maintainer: [-Ubuntu MOTU Developers <email address hidden>-]
[-Original-Maintainer:-] RISKO Gergely <email address hidden>            
Version: [-1.01+20060420-1-] {+1.01+20060420-1ubuntu0.1~ppa1i+} 

===========
$ debdiff libhdate-dev_1.4.11-1_i386.deb ../../intepid/libhdate-dev_1.4.11-1ubuntu0.1~ppa1i_i386.deb 
File lists identical (after any substitutions)            

Control files: lines which differ (wdiff format)
------------------------------------------------
Depends: libhdate1 (= [-1.4.11-1)-] {+1.4.11-1ubuntu0.1~ppa1i)+}
Maintainer: [-Ubuntu MOTU Developers <email address hidden>-]
[-Original-Maintainer:-] Debian Hebrew Packaging Team <email address hidden>                                                           
Version: [-1.4.11-1-] {+1.4.11-1ubuntu0.1~ppa1i+} 

===========
$ debdiff libhdate-pascal_1.4.11-1_i386.deb ../../intepid/libhdate-pascal_1.4.11-1ubuntu0.1~ppa1i_i386.deb
[The following lists of changes regard files as different if they have
different names, permissions or owners.]

Files in second .deb but not in first
-------------------------------------
-rw-r--r--  root/root   /usr/lib/fpc/2.2.2/units/i386-linux/hdate/Package.fpc
-rw-r--r--  root/root   /usr/lib/fpc/2.2.2/units/i386-linux/hdate/fpc_README
-rw-r--r--  root/root   /usr/lib/fpc/2.2.2/units/i386-linux/hdate/hdate.o
-rw-r--r--  root/root   /usr/lib/fpc/2.2.2/units/i386-linux/hdate/hdate.ppu
-rw-r--r--  root/root   /usr/lib/fpc/2.2.2/units/i386-linux/hdate/hdate_dyn_pascal.o
-rw-r--r--  root/root   /usr/lib/fpc/2.2.2/units/i386-linux/hdate/hdate_dyn_pascal.ppu

Files in first .deb but not in second
-------------------------------------
-rw-r--r--  root/root   /usr/lib/fpc/2.2.0/units/i386-linux/hdate/Package.fpc
-rw-r--r--  root/root   /usr/lib/fpc/2.2.0/units/i386-linux/hdate/fpc_README
-rw-r--r--  root/root   /usr/lib/fpc/2.2.0/units/i386-linux/hdate/hdate.o
-rw-r--r--  root/root   /usr/lib/fpc/2.2.0/units/i386-linux/hdate/hdate.ppu
-rw-r--r--  root/root   /usr/lib/fpc/2.2.0/units/i386-linux/hdate/hdate_dyn_pascal.o
-rw-r--r--  root/root   /usr/lib/fpc/2.2.0/units/i386-linux/hdate/hdate_dyn_pascal.ppu

Control files: lines which differ (wdiff format)
------------------------------------------------
Depends: [-fp-compiler (>= 2.0.0),-] libhdate1 (= [-1.4.11-1)-] {+1.4.11-1ubuntu0.1~ppa1i), fp-compiler (>= 2.0.0)+}
Maintainer: [-Ubuntu MOTU Developers <email address hidden>-]
[-Original-Maintainer:-] Debian Hebrew Packaging Team <email address hidden>
Version: [-1.4.11-1-] {+1.4.11-1ubuntu0.1~ppa1i+}

===========
$ debdiff libhdate-perl_1.4.11-1_i386.deb ../../intepid/libhdate-perl_1.4.11-1ubuntu0.1~ppa1i_i386.deb
File lists identical (after any substitutions)

Control files: lines which differ (wdiff format)
------------------------------------------------
Depends: {+libhdate1 (= 1.4.11-1ubuntu0.1~ppa1i), perl (>= 5.10.0-11.1ubuntu2),perlapi-5.10.0,+} libc6 (>= 2.4), libgcc1 (>= [-1:4.1.1-21), libhdate1 (= 1.4.11-1),-] {+1:4.1.1),+} libstdc++6 (>= [-4.1.1-21), perl (>= 5.10.0-9), perlapi-5.10.0-] {+4.1.1)+}
Maintainer: [-Ubuntu MOTU Developers <email address hidden>-]
[-Original-Maintainer:-] Debian Hebrew Packaging Team <email address hidden>
Version: [-1.4.11-1-] {+1.4.11-1ubuntu0.1~ppa1i+}

===========
$ debdiff libhdate-python_1.4.11-1_i386.deb ../../intepid/libhdate-python_1.4.11-1ubuntu0.1~ppa1i_i386.deb
File lists identical (after any substitutions)

Control files: lines which differ (wdiff format)
------------------------------------------------
Depends: [-libc6 (>= 2.4), libgcc1 (>= 1:4.1.1-21),-] libhdate1 (= [-1.4.11-1),libstdc++6 (>= 4.1.1-21), python (>= 2.5),-] {+1.4.11-1ubuntu0.1~ppa1i),+} python (<< 2.6), {+python (>= 2.5),+} python-central (>= [-0.6.6)-] {+0.6.7), libc6 (>= 2.4), libgcc1 (>= 1:4.1.1), libstdc++6 (>= 4.1.1)+}
Maintainer: [-Ubuntu MOTU Developers <email address hidden>-]
[-Original-Maintainer:-] Debian Hebrew Packaging Team <email address hidden>
Version: [-1.4.11-1-] {+1.4.11-1ubuntu0.1~ppa1i+}

===========
$ debdiff m-tx_0.60c-1_i386.deb ../../intepid/m-tx_0.60c-1ubuntu0.1~ppa1i_i386.deb
File lists identical (after any substitutions)

Control files: lines which differ (wdiff format)
------------------------------------------------
Installed-Size: [-884-] {+888+}
Maintainer: [-Ubuntu MOTU Developers <email address hidden>-]
[-Original-Maintainer:-] Roland Stigge <email address hidden>
Recommends: [-musixlyr (>= 1.10-4),-] musixtex (>= 1:0.98-1), {+musixlyr (>= 1.10-4),+} pmx (>= 2.1.0-1)
Version: [-0.60c-1-] {+0.60c-1ubuntu0.1~ppa1i+}