Here's the scoop on Flash from a developer's perspective...
Flash should never be relied on as the only visual medium on the site as it is not supported on all platforms and is often blocked by corporate and personal firewalls. This means, as jessica wisely suggested, you should always provide an HTML alternative bearing in mind Flash does not corner the market as to high quality graphics and rich web 2.0 content. Flash is not compatible with accessibility features in browsers and by W3C standards. As a plugin is required, this may inconvenience those users who do not support it by default and are prompted to download, not to mention many office LAN's prevent users from installing software of this type on workstations. Meanwhile, a quality web designer with few HTML skills can create photo gallery oriented sites that are both usable, cross browser compatible and fully accessible using many IDE's and content management systems in place today with open source third party modules and frameworks to help them.
HTML5 or whatever the W3C ends up doing as the next incarnation of HTML and CSS will, in the future, make embedding objects and media far easier than it ever was in the past. Even Flash used for progressive streaming of videos in the "You Tube" way will likely erode in favor to developers over time.
Combine this with the fact many PDA's and mobile web platforms don't support Flash right now, so you're losing a valuable demographic of users right there.
The general advice these days is not to "rely" on it and avoid splash screens that utilize Flash. Use it for complimentary add-ons but also provide an HTML alternative, i.e. create a nice slideshow option in Flash format but for people with Flash plugin missing display a nice HTML version that uses, say, MooTools or LightBox or one of the many other DHTML open source frameworks to extend a PHP based/database driven gallery as well.
My .02
-jim