I am not sure how it would affect current/existing clients, but I believe switching to gpl would possibly stifle ArtForz's estimated ~25% dominance of block generations and total Bitcoin balance.
Total Bitcoins as of now: 3914550
If ArtForz had 25% of total Bitcoins, then: 978637.5
If each Bitcoin is worth $0.06, then total value: $58,718.25
These figures are not reliable, but are an example.
Here's a better reference:
7.28.2010: <ArtForz> for my opencl sha256 code the 5770 is only 5% faster than the 4870 per core-mhz
block 70800 1280322359
# date -d "@1280322359"
Wed Jul 28 08:05:59 CDT 2010
So estimated start time of ArtForz's opencl client was at block 70,800. Current block count is 78,292. 7,492 blocks.
7,492 * 50btc = 374,600btc.
374,600btc * 25% = 93,650btc
93,650btc * $0.06 = $5,619.
So, it is fairly safe to conclude that ArtForz has approximately ~$5,600ish value (and will most likely increase in value over time)
If they were to sell all bitcoins right now on open exchange markets, they could get a guaranteed amount of ~$2,348.
$0.0616 each for BTC757.156 ($46.6408096) at mtg.
$0.0615 each for BTC500 ($30.75) at mtg.
$0.0609 each for BTC610.6 ($37.18554) at mtg.
$0.0601367 each for BTC936 ($56.2879512) at mtg.
$0.0601366 each for BTC2049 ($123.2198934) at mtg.
$0.0601 each for BTC100 ($6.01) at mtg.
$0.06 each for BTC525 ($31.5) at mtg.
$0.0597 each for BTC25.963 ($1.5499911) at mtg.
$0.0596622 each for BTC879 ($52.4430738) at mtg.
$0.0592 each for BTC96.3 ($5.70096) at mtg.
$0.059 each for BTC20 ($1.18) at mtg.
$0.059 each for BTC1000 ($59) at bcm.
$0.0585 each for BTC200 ($11.7) at mtg.
$0.0581 each for BTC600 ($34.86) at mtg.
$0.058 each for BTC1.207 ($0.070006) at mtg.
$0.0574999 each for BTC515.4 ($29.63544846) at mtg.
$0.0574215 each for BTC2404 ($138.041286) at mtg.
$0.057 each for BTC200 ($11.4) at bcm.
$0.056 each for BTC1000 ($56) at bcm.
$0.055 each for BTC100 ($5.5) at bcm.
$0.05148 each for BTC2404 ($123.75792) at mtg.
$0.051 each for BTC500 ($25.5) at mtg.
$0.0505 each for BTC1000 ($50.5) at bcm.
$0.0501 each for BTC400 ($20.04) at bcm.
$0.05 each for BTC100 ($5) at bcm.
$0.05 each for BTC100 ($5) at bcm.
$0.05 each for BTC100 ($5) at mtg.
$0.05 each for BTC1000 ($50) at bcm.
$0.05 each for BTC1000 ($50) at bcm.
$0.05 each for BTC1000 ($50) at bcm.
$0.04825 each for BTC2404 ($115.993) at mtg.
$0.0472 each for BTC1000 ($47.2) at bcm.
$0.045 each for BTC100 ($4.5) at bcm.
$0.045 each for BTC200 ($9) at bcm.
$0.044 each for BTC200 ($8.8) at bcm.
$0.04 each for BTC500 ($20) at bcm.
$0.04 each for BTC600 ($24) at mtg.
$0.04 each for BTC1600 ($64) at bcm.
$0.04 each for BTC2000 ($80) at bcm.
$0.039 each for BTC500 ($19.5) at mtg.
$0.038 each for BTC500 ($19) at mtg.
$0.0375 each for BTC200 ($7.5) at bcm.
$0.037 each for BTC500 ($18.5) at mtg.
$0.035 each for BTC200 ($7) at bcm.
$0.035 each for BTC500 ($17.5) at mtg.
$0.035 each for BTC3000 ($105) at bcm.
$0.034 each for BTC259.118 ($8.810012) at mtg.
$0.033 each for BTC500 ($16.5) at mtg.
$0.033 each for BTC1000 ($33) at bcm.
$0.032 each for BTC500.001 ($16.000032) at mtg.
$0.0301 each for BTC300 ($9.03) at bcm.
$0.03 each for BTC21 ($0.63) at mtg.
$0.03 each for BTC100 ($3) at bcm.
$0.03 each for BTC100 ($3) at bcm.
$0.03 each for BTC500 ($15) at bcm.
$0.03 each for BTC1000 ($30) at bcm.
$0.022 each for BTC1000 ($22) at bcm.
$0.0202 each for BTC1000 ($20.2) at bcm.
$0.02 each for BTC100 ($2) at bcm.
$0.02 each for BTC500 ($10) at bcm.
$0.02 each for BTC1000 ($20) at bcm.
$0.02 each for BTC1000 ($20) at bcm.
$0.02 each for BTC1000 ($20) at bcm.
$0.02 each for BTC1000 ($20) at bcm.
$0.015 each for BTC1000 ($15) at bcm.
$0.0121 each for BTC5000 ($60.5) at bcm.
$0.011 each for BTC5000 ($55) at bcm.
$0.01 each for BTC1000 ($10) at bcm.
$0.01 each for BTC1000 ($10) at bcm.
$0.01 each for BTC1000 ($10) at bcm.
$0.01 each for BTC1000 ($10) at bcm.
$0.01 each for BTC2000 ($20) at bcm.
$0.01 each for BTC5000 ($50) at bcm.
$0.0096 each for BTC1000 ($9.6) at bcm.
$0.007 each for BTC100 ($0.7) at bcm.
$0.006 each for BTC10000 ($60) at bcm.
$0.005 each for BTC1000 ($5) at bcm.
$0.005 each for BTC1000 ($5) at bcm.
$0.005 each for BTC5000 ($25) at bcm.
$0.005 each for BTC8542.255 ($42.711275) at bcm.
Total amount for BTC93650 is $2348.64719856