Attach meshes to last selected, keeping pivots, transforms and hierarchy. Remake of script by Thomas Hamilton from here http://www.creativecrash.com/maya/script/attachmaster #WIP

PS_Attach.py

import maya.cmds as mc
import string


		
def PS_Attach( objs ):
		
	def dag_par( obj ):
		if isinstance( obj, basestring ):
			return string.join( mc.listRelatives( obj, pa=1, f=1 )[0].split('|')[:-1], '|' )
	
	pre_sel = None
	if objs:
		pre_sel = mc.ls( objs, tr=1, l=1 )
	else:
		pre_sel = mc.ls( sl=1, tr=1, l=1 )

	sel = []
	for s in pre_sel:
		if mc.filterExpand( mc.listRelatives( s, s=1, pa=1, f=1 ), sm=12 ): sel.append( s )
	if sel:
		if len( sel ) < 2 : 
			mc.error( 'Select at least two polygon objects!' )
	else:
		mc.error( 'No polymeshes selected!' )	
	
	par =  dag_par( sel[-1] ) # store last mesh dag path
	origname = par.split('|')[-1] # store short name of last mesh
	
	mat  = mc.xform( par, q=1, ws=1, m=1 ) # store transform of last mesh
	rpvt = mc.xform( par, q=1, ws=1, rp=1 )
	spvt = mc.xform( par, q=1, ws=1, sp=1 )

	dd = [] # find any selected meshes above last in hierarchy
	for s in sel[:-1]: 
		pd = dag_par( s )		
		if par.find( pd ) != -1 and par != pd : dd.append( pd )
		
	if dd: # get topmost mesh above last
		min = 999
		new_par = None
	
		for s in dd:
			l = len( s.split('|') )
			if l < min : 
				min = l
				new_par = s
		par = new_par  	
		
	par = dag_par( par ) # swap last and topmost
	id = sel.index( par )
	if id:
		tmp = sel[ id ]	
		sel[ id ] = sel[ -1 ]	
		sel[ -1 ] = tmp
				
	chn = [] # children to unparent

	sel = [ s.split('|')[-1] for s in sel ] # convert dag to short names
	par = string.join( par.split('|')[:-1], '|' ) # parent of output mesh
		
	for s in sel:
		
		shapes = mc.listRelatives( s, s=1, pa=1 ) # remove extra shapes
		if len( shapes ) > 1 : 
			mc.delete( shapes[1:] )
			
		ch = mc.listRelatives( s, c=1, typ='transform' ) # unparent all children from selected meshes
		if ch : 
			for c in ch: 
				if not c in sel:
					chn.append( c )
					mc.parent( c, w=1 )
			
		if mc.listRelatives( s, p=1 ) : 
			mc.parent( s, w=1 ) # unparent all selected meshes
		
	nullGrp = mc.group( em=1 )	# Null group to handle transform
	mc.xform( nullGrp, ws=1, m=mat )
	
	comb = mc.polyUnite( sel, ch=0 ) # Merge meshes	

	mc.xform( comb, ws=1, rp=rpvt ) # restore transform
	mc.xform( comb, ws=1, sp=spvt )
	mc.parent( comb, nullGrp )
	mc.makeIdentity( comb, a=1, t=1, r=1, s=1, n=0 )
	mc.parent( comb, w=1 )
	
	mc.delete( nullGrp )
	
	if par : mc.parent( comb, par ) # Parent everything back
	if chn : mc.parent( chn, comb )
	
	mc.select( mc.rename( comb, origname ), r=1 )
	
	mc.setAttr( origname + '.displayRotatePivot', 1 )
	mc.setAttr( origname + '.displayScalePivot', 1 )

PS_Attach( mc.ls( sl=1 ) )

Hi there,
The script is very handy, thanks a lot!
I have just one idea- is it possible to add ability to combine instance-type meshes, without breaking up connection between them?
I mean, in 3ds max and blender it's works just by default- you can easily attach some new geometry parts to one of the instances and all other will update permanently; but in maya if you try to attach some new mesh to one of the instance-type object all other instances just disappear =)