IAC will not allow offline bouncing and will not have sample accurate timing, so it's not so useful in practice unless you can live with those limitations.
As far as DAWs go, I can recommend Cubase and Nuendo, which work perfectly with all the features of VE Pro 5. Pro Tools also works well, but is a less efficient plugin host in general. Since Cubase and Nuendo can use the VST3 plugin and Pro Tools the RTAS plugin, multiple MIDI ports are already available without the need for a workaround.
In Logic you can use the Event/Audio Input plugin if you're not planning on doing any further heavy processing in Logic (this is due to the necessity for Live mode, explained in the manual). The old way of simply having a Server Interface plugin for every 16 MIDI channels still works for having more MIDI ports (at the expense of having more connected instances). The Multiport Environment tends to work well when no MIDI CC are involved, because it seems that that is when Logic starts dropping events. We can only hope that Apple fixes this issue. Currently there is also a problem when selecting the 5.1 or 6.1 versions of the Server Interface plugin in that Logic will assign 5.0 or 6.0 instead as channel configuration. This should be fixed in Logic 9.1.7 however.
In Digital Performer dynamic latency compensation currently does not work for AU instruments, but this will be fixed by MOTU in the soon to be released Digital Performer 8. Also, offline bouncing does not work for the Event/Audio Input plugins there. It is however possible to freeze first before bouncing as a workaround. It's unsure at this stage whether offline bouncing will ever work for the Event/Audio Input plugins, but we're in contact with MOTU on the issue.