Lennyjoe, purchasing the NFA item out of your separate account creates two different issues:
1) The first is any tax consequences of funding a trust. A trust is still a separate taxable entity. In addition to state tax issues that can crop up, any non-charitable gift of more than the gift tax exemption to a single entity can trigger the federal estate tax. Currently the gift tax exemption is $10,000 plus a variable inflation adjustment allowance. So if you bought say an M16 registered receiver or bought a bunch of suppressors in one year, you could end up reducing the estate tax exemption after your death. For a lot of people who want an SBR or suppressor, this is not a big issue; but it certainly will effect guys who can afford registered machineguns.
If you are funding the trust randomly out of your personal account it makes it harder to separate your purchases and trust purchases for tax purposes. People are more prone to making mistakes in that situation - and since purchasing an item for your trust from your personal account and then failing to report the gift to the trust looks a lot like tax evasion to some auditors, it can create unnecessary problems.
2) The second issue is that the NFA firearm is registered to the Trust - not the Grantor of the Trust. Having a separate bank account in the Trust's name and using that for all of your purchases and tax stamp fees helps build a nice solid chain of evidence if you ever have to go into court about whether your possession of an NFA item is legal.
Also, in the past in many states, there has been a lot of litigation over whether a trust was a "sham" trust designed to circumvent state probate law or a legitimate trust that served what the state saw a legitimate purpose. As revocable living trusts have become more popular, that has become less of an issue. I would guess that purchasing and paying tax stamps out of your personal account could conceivably cause some heartache in those states though since it might indicate a blurring between the Grantor and the Trust as separate entities. Just a guess though... it would require a lot of legal research to give a solid answer to that particular issue (sham trusts vs. legitimate trusts) and I'm guessing if you don't want to pay an attorney for a gun trusts, you sure don't want to pay one for that kind of research
This thread actually inspired me to do an article on your question and it includes an analysis of the gun trust greyling22 posted and how well it works as a Texas gun trust. When I get the article finalized in a week or so, I will probably post it here (if you want to wait).