Just for the record: I upgraded to ESX 3.0.2 and...
1.) So far, I did not observe any stalls on the emulated e1000 (em) interfaces. Currently I am playing with the vmxnet driver as well. 2.) VGT mode seems to work correct, very short ethernet frames (i.e., ICMP ping packets produced by windows XP machines routed over a virtual OpenBSD 4.1 firewall with interfaces in VGT mode) are not dropped anymore. Thanks for all the feedback.